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BEGINNINGS: THE VIEW FROM SCIENCE
Our times represent only the latest in a long series of layers of human history dating back some 5,000 years, when someone figured out that the signs and symbols of the bureaucrats in places Egypt and Mesopotamia could be put to a more abstract, creative use. Historical records only date back a small part of the span of time which separates us from the earliest members of our species, and the whole panoply of human ancestors since our divergence with the chimpanzees is but a drop in the ocean of life. Advanced life forms, at least those whose affiliations we can trace to some extent, date back to the Ediacaran and Cambrian periods, some 550-500 million years ago or thereabouts, and this is a mere eighth or so of the story of life on this planet. Furthermore, the earth itself and the solar system which houses it is a relative parvenu on the universal scene.
Yet, though we have existed for what is, relatively speaking, this miniscule amount of time, our species has made great (bipedal) strides forward - including a giant leap to the moon and back, flying higher than any other species despite not having wings. We have also, somehow, managed to figure out that our life on earth is the briefest flicker of a candle in the vast darkness of time and space.
This section briefly outlines the journey from Big Bang to First Man in a series of stages: -
THE UNIVERSE: A BRIEF AND CURSORY HISTORY
IN THE BEGINNING...
The universe in the minute amounts of time immediately after the Big Bang was a very active place. The first picosecond saw the coming and going of the Planck epoch (tp, 10-43 seconds), a truly bizarre fraction of a moment during which the laws of physics as we know them seem not to have applied, at the end of which the first fundamental interaction, namely gravity, emerged.
This was followed in turn by the grand unification epoch, before the separation of the electronuclear force and emergence of the three remaining fundamental interactions in order: -
Electronuclear force
Strong nuclear force
Electroweak force
Weak nuclear force
Electromagnetism
THE INFLATIONARY EPOCH
WHAT'S THE MATTER?
Quark
10-12 to 10-6 s.
Hadron
10-6 to 1 s.
Lepton
1 to 10 s.
Photon
10 s. to 377 k.y.
Shortly after this time, matter and antimatter appear in the form of the various types of subatomic particle-antiparticle pairs. The annihilation of much of the content of the universe in the form of matter-antimatter reactions left a residue of leftover matter, which laid the foundations for the universe we see today.
The most primitive and earliest of these were quarks and antiquarks, which formed a quark-gluon plasma, until the cooling of the universe enabled baryogenesis to take place, leading to the first baryons (subatomic particles usually made of three quarks, such as the proton and neutron) and mesons (made up of an even number of quarks) forming. Baryons and mesons are collectively referred to as hadrons, which were the dominant form of matter until about a second after the Big Bang. At this point, neutrino decoupling took place, leaving the cosmic neutrino background. Neutrinos are another species of elementary particle, one of the class of fermions which rarely interact with matter, hence most of the neutrinos which appeared at this time remain unchanged. Also possibly making a first appearace at this time are primordial black holes. The end of the hadron epoch is also marked by the mutual annihilation of most of the hadrons and antihadrons, leaving the nascent universe dominated by leptons, another group of elementary particles which include the electron. Again, the leptons and antileptons annihilate one another by the end of this period, leaving photons as the dominant particle for a considerable time span thereafter.
Indeed, photons dominated the universe during a period between 10 seconds and 377,000 years after the formation of the universe, during which the universe was predominantly filled with a photon-baryon fluid. The first nuclei form between three and 20 minutes after the Big Bang, with protons (hydrogen ions) combining by means of nuclear fusion to form helium nuclei, until the rapid cooling put a stop to this process. At this point, the photons are unable to travel far, thus filling the cosmos with a dense, opaque plasma.
Further cooling takes place until about 47,000 years after the universe forms, by which stage cold dark matter, in the form of the atomic nuclei, comes to dominate over relativistic radiation, comprised of photons.
100,000 years in, the first molecules - helium hydride - form. These would eventually react with hydrogen to form molecular hydrogen, which would in turn provide the fuel needed for the formation of stars.
RECOMBINATION AND DECOUPLING
THE FIRST DARK AGE
Recombination and decoupling resulted in the universe becoming filled with a brilliant pale orange glow, which lasted for about 3 million years, before this "light" redshifted into a non-visible spectrum (i.e. its wavelength became longer, moving it into the infrared band).
A GOLDILOCKS ZONE?
About 10 to 17 million years after its formation, the average temperature of the universe was somewhere between 0 and 100°C - the temperature at which H2O is in liquid form. This has led to some speculation that, in some portion of the universe, conditions may have been suitable for the formation of rocky planets, bathed in the warmth of the universe as a whole, which may in turn have provided the opportunity for life to emerge.
FIAT LUX!
About 200-500 million years after the universe came to be, the universe again burst into light with the formation of the first stars and galaxies. By this stage, those ripples which formed during the first moments of the universe had developed into dark matter filaments, to which the earliest large structures became drawn, eventually leading to the clusters and superclusters of galaxies which fill the universe today.
These first stars were not, however, like our own sun: they were enormous, between 100 and 300 solar masses and, having formed from the combination of hydrogen and helium hydride, possessed none of the metallic spectra which most stars today possess. Additionally, due to their size, these stars were extremely unstable and, as such, had much shorter lifespans than their successors, exploding after only a few million years as pair-instability supernovae, which in turn yielded the heavier elements which are found in the universe - as well as in you and I - today.
REIONIZATION
THE STELLIFEROUS ERA
After about a billion years, reionization had completed its work and the universe took on an appearance roughly the same as it is today. This is known as the "Stelliferous Era," as it is the combined luminosity of the stars which shine like a beacon in the darkness of space. Thus far three distinct populations of stars have been identified. The first of these, known as Population III, is represented by the very first stars, the supermassive metal-free stars which appeared during the Dark Ages.
These gave rise to Population II, which includes the oldest stars visible via astronomy. HE 1523-0901, which is, at some 13.2 billion years old, the oldest star in the Milky Way; Cayrel's Star (alias the less-snappy BPS CS31082-0001), around 12.5 billion years old; Sneden's Star (BPS CS22892-0052); and BD +17° 3248, perhaps even older than HE 1523-0901. These stars are defined by the paucity of metals contained within. 2MASS J18082002−5104378 B, part of a binary system, is probably the oldest star yet discovered, coming in at a venerable 13.53 billion years of age.
By about 4 billion years after the Big Bang, Population I - or metal-rich - stars begin to form. Our sun is an intermediate member of this group: some stars, such as μ Arae, possess a greater proportion of metallic elements. Between Population II and Population I can be placed stars located among the intermediary disc population.
THE BIRTH OF THE SOLAR SYSTEM
Within the Orion Spur of the Milky Way galaxy, some 4.6 billion years ago, material within a molecular cloud or "stellar nursery" began a process of gravitational collapse. Much of the mass gravitated to the centre, with a circular disk forming in orbit around it. This would give rise in relatively short order to the sun and its system of planets, asteroids and sundry other objects.
THE SOLAR SYSTEM: ORIGIN AND INFANCY
SOMEWHERE IN A GALAXY...
PLANETARY FORMATION
Close to the sun, a series of planetesimals formed, composed of metals such as iron and aluminium, as well as silicates, growing to about 200 times smaller than the earth (0.05 M⊕) by about 4.59 billion years ago.
Further out, beyond the "frost line," beyond which volatile icy compounds are able to maintain a solid state, other planetesimals were forming, reaching sizes of up to four times that of the earth within some 3,000,000 years. From these would develop the greatest planets of our solar system, the gas giants Jupiter and Saturn and, further out, the ice giants Uranus and Neptune.
The rapid increase in mass and size of the two gas giants had repercussions for the inner Solar System during this time: evidence from Ceres, a dwarf planet in the asteroid belt, suggests that, as Jupiter grew - and possible moved slightly inward towards the sun - it caused significant preturbations in the orbits of objects both inside and outside its orbit, leading to a Jovian Early Bombardment. Possibly added to this were the effects of Saturn's undergoing a similar growth, leading to the two planets, coming into orbital resonance, contributing to a Primordial Heavy Bombardment (about 4.568 billion years ago or shortly afterwards). Jupiter's growth also resulted in the dynamic excitation and clearing of the Asteroid Belt, leaving less than 1% of the original planetary embryos intact at around 3 AU. By this stage, the last remnants of nebular gas were dispersed.
AND THE EARTH WAS WITHOUT FORM AND VOID
The inner Solar System was a congested place just over 4.5 billion years ago, with between 50 and 100 protoplanets (large planetary embryos), ranging in size from about that of the moon to that of Mars. These were often given to violent interactions and collisions: one such catastrophe likely ripped the outer envelope of Mercury, leaving the chthonian planet-like world which exists today.
HOW TO SOLVE A PROBLEM LIKE THEIA
Another such coming together seems to have happened to the earth during its formative period: a Mars-sized object, given the name Theia seems to have careered into the earth, causing in turn the expulsion of masses of material which would eventually form the moon.
The earth itself is most widely held to have formed by about 4.54 billion years ago, by which stage it had more or less cleared its orbit of other planetary embryos (with the exception of the Earth trojan Theia mentioned above). The moon was formed by 4.533 billion years ago. In terms of geology sensu stricto (i.e. covering the earth only), the Hadean Eon is held to begin around 4.54 billion years ago, with the Pre-Nectarian period on the selenological timescale beginning at 4.533 billion years ago and the areological Pre-Noachian beginning with the formation of Mars in about 4.5 billion years ago.
LATE HEAVY BOMBARDMENT
YOU ARE HERE
While the possibility of life having once existed on Mars or even Venus, not to mention the possibility of it still clinging on on a remote moon of Jupiter or Saturn, this is strictly beyond the purview of this series. From now on, we will concentrate on developments taking place on our beautiful planet earth.
ALIVE! 1: THE RISE OF LIFE
ORIGINS
Deep within the newly-formed oceans of the earth, clustered around fissures on the seabeds known as hydrothermal vents, which emit superheated water, enriched with a wide variety of minerals, from below the earth's outer layer or crust, there is life - and a lot of it. The discovery of these colonies, formed from bacteria, invertebrates and even fish, has led to a very plausible suggestion that the first life on earth emerged around one or more of these "black smokers."
The precise make-up of the earliest life, however, remains something of a mystery. One of the major attributes common to all life we know of on the planet is, of course, our DNA (or, to give the molecule its full name, deoxyribonucleic acid). DNA, of course, forms the iconic "double helix" structure and is a molecule of not insignificant complexity. This has led to another plausible suggestion: as DNA has a less-complex cousin - still active and important in our cells to this day - known as RNA (ribonucleic acid), then perhaps RNA appeared and played an important role at the dawn of life. This RNA world hypothesis posits that self-replicating RNA molecules were present on earth very early on, along with amino acids (the building blocks of proteins). Eventually, amino acids combined to form more complex molecules (peptides, the shortest of which, dipeptides include two amino acids joined by a single peptide bond; thereafter, oligopeptides, polypeptides and eventually proteins "proper"). This in turn would likely have led to a transitional stage in the development of life, from an RNA world to an RNP world, with ribonucleoproteins composed of RNA and RNA-binding protein.
ENTER DNA
Perhaps by about 4-3.5 billion years ago, this primitive world of RNA and RNP-based life forms had progressed, producing the first organisms whose genetic coding was based on DNA.
However, this need not necessarily have been DNA as we know it: Peter Ward, who developed the Medea Hypothesis - which puts forth the idea that life is inherently self-destructive - notes that laboratory experiments have succeeded in producing various "exotic" takes on DNA, proposing that: -
Ward also suggests that the advent of DNA - specifically "our" DNA - led, in addition to the demise of the less-complex RNA & RNP life, to the sounding of the death-knell for any other types of life which were around at the time, adding that: "the greatest mass extinction of all may have been the first, the DNA (as we have it now, anyway) incurred mass extinction."
OUR TINY ANCESTORS
From this starting point, life developed over time, though appears to have remained rather simple throughout the Archean Eon. The last universal common ancestor (LUCA) likely lived during this time, with dates ranging from about 3.8 to shortly before 2.5 Gya (billion years ago), marking the split between the two domains of prokaryotes (single-selled organisms lacking membrane-bound organelles): bacteria and archaea. Early colonies formed microbial mats which, over time, led to the formation in some areas of stromatolites: some studies have suggested that stromatolites were already forming during the period in question. By about 3.22 Gya, these microbial mats may have extended out of the seas and onto land.
The earliest life forms during this period would have been very different from life as we know it, and may have produced methane: indeed, already by about 3.465 Gya, according to J. William Schopf and colleagues, a number of different lifestyles were utilised, including forms similar to present-day methanogenic (methane-producing) archaea and methanotrophic (methane-consuming) bacteria, among eleven microbial fossils found in the Apex chert in Australia. Already, by this stage, however, this methane production may have led to global cooling: James Kasting suggests that the output of these life forms led to a methane haze (the earliest clouds) forming which reflected light from the sun back out into space, leaving a cold buffer on the earth's surface, around 3.7 Gya.
PHOTOSYNTHESIS
Among the other microbes identified by Schopf et al in the Apex chert were two species with affinities with "extant phototropic bacteria": these primitive photosynthesizers (if this identification is accepted at this early date) would gain in significance throughout the remainder of the Archean: oxygenation may already be implicated in the Pongola glaciation, which took place around 2.9 Gya. Somewhere in the region of 2.7 Gya, a new group of photosynthetic organisms, the cyanobacteria, had evolved, and, by 2.5 Gya, these stromatolite-forming shallow water inhabitants were well on the way to changing the world forever, thanks to their novel lifestyle, which involved converting water and carbon dioxide (the latter primarily of volcanic origin, then as now) into hydrocarbons, also creating oxygen as a byproduct.
This led in turn to a much-increased atmospheric level of oxygen, with a commensurate lowering of the amount of methane smog present, and additionally caused the oxidization of the iron in the then-metal-rich oceans.
GREAT OXIDATION EVENT
By the beginning of the Paleoproterozoic era 2.5 Gya, atmospheric oxygen was continuing to rise. By 2.4 Gya, this had become a crisis for life, almost certainly leading to one of the most significant extinction events, as well as the first "Snowball Earth" in the form of the Huronian (or Makganyene) glaciation, which lasted until about 2.1 Gya.
MULTICELLULAR 1: FRANCEVILLE
The after-effects Great Oxygenation Event heralded some significant changes in the nature of life on earth. It is likely that, among the organism which took to life in the new, oxygen-rich world, were the eukaryotes. Aided by their possessing mitochondria (which may have originally represented a separate, symbiotic species), it is also possible that eukaryotes became the first macroscopic organisms in the period immediately after the GOE and Huronian glaciation: Grypania, an engmatic spiral-shaped creature which first appears in the fossil record about 2.3 Gya, is thought by some to be an alga, while Diskagma buttonii, a fungus or fungus-like organism, was present - on land - by 2.2 Gya.
Most remarkable of all is a rich biota found in rocks dating to about 2.1 Gya from Franceville in Gabon. Known as the Gabonionta or Francevillian biota, these finds include several different forms, some up to 6.7 inches in size, and interpreted as having been capable of lateral and vertical movement. The Gabonionta likely lived in the shallows near a river delta and probably fed on the microbial mats also evidenced at this location.
The Franceville experiment was, however, short-lived: no evidence of their survival has been found in later strata at Franceville and it has been proposed that, having radiated in the wake of the Lomagundi event, a 130-250 million year period of time during which very high levels of oxygen and carbon were available, offering an opportunity for complexity, they eventually fell victim to a subsequent reduction in oxygen in their ecosystem.
Some life may have sought to escape this anoxic ocean onto land: hairpin-shaped and discoid fossils discovered in the Stirling Range in Australia, dating from about 1.9 Gya, possibly represent early life outside of the seas.
A "BORING BILLION?"
Some time after the Gabonionta came and went, the earth entered what Martin Brasier termed the "Boring Billion." This period of time is marked by considerable stability in terms of evolution, environment and geodynamics. The oceans, according to a theory put forward by Donald Canfield, had come to be anoxic and sulphidic, i.e. filled with dissolved hydrogen sulphide (a similar pattern reasserted - indeed, reasserts - itself throughout the story of life in low-oxygen bodies of water). This model of the Canfield ocean suggests that the oxygenating effects of the GOE were transient, and that deep water remained anoxic throughout, remaining so for a long time afterwards. The ocean depths became stagnant and anaerobic, leading to the sulphidification of the seas and preventing the deposition of banded iron formations which are common in strata dating to between 2.4 and 1.8 Gya and again after 750 mya, but rare in the interim.
SEX AND PREDATION
Macroscopic fossils continue to appear sporadically during the Mesoproterozoic, with "decimetre-scale multicellular eukaryotes" dating to about 1.56 Gya present in the Gaoyuzhuang Formation in northern China, described by Shixing Zhu and colleagues in 2016. These "have statistically regular linear to lanceolate shapes up to 30 cm long and nearly 8 cm wide, suggesting that the Gaoyuzhuang fossils record benthic multicellular eukaryotes of unprecedentedly large size."
Eukaryotes appear to become dominant between 1.25 and 1 Gya: stromatolites peak by the earlier date, only to decline in diversity and abundance thereafter, with acritarchs undergoing an increase by the latter date. Acritarchs also grew larger, more complex and developed larger spines in a greater number, perhaps suggesting the advent of predation, likely by protist herbivores. Cyanobacteria were still more abundant than algae around 1.1 Gya, at which point we have evidence from the Taoudeni Basin in Mauritania of porphyrins, the molecular fossilised forms of chlorophylls.
Another landmark was achieved by 1 Gya: Bangiomorpha pubescens, a red alga living between 1.06 and 1.03 Gya and present in the Hunting Formation on Somerset Island, Canada yields the earliest evidence of sexual reproduction.
THE FIRST LAND PLANTS
THE ANIMALS APPEAR
Around 950 mya, the evolutionary path leading to animals was being forged by tiny creatures bearing some similarities to modern choanoflagellates. Around 760 mya, Otavia antiqua, possibly the earliest animal fossil, appears.
SNOWBALL EARTH!
Around 750 mya, the first of a series of cooling events is detected: this is the Kaigas glaciation, which prefigured the longer-lasting glacial events during the succeeding Cryogenian period. Of these, the Sturtian glaciation lasted from about 717 to 680 mya, with the Marinoan following between about 650 to 635 mya. Evidence suggests that the demosponges, featuring a skeleton formed of calcium carbonate, arose just prior to the later event.
MULTICELLULAR 2: ENIGMATIC EDIACARANS
Two shorter glacial periods, the Gaskiers (c.579.88-579.63 mya) and Baykonurian (c.547-541 mya) took place in the subsequent Ediacaran period. Between these two cold snaps, a remarkable event occurred, known as the Avalon radiation, which produced a whole host of mysterious fossils, many of them seemingly unrelated to the Cambrian biota which emerged during the Phanerozoic.
The earliest and longest-lasting of the three major assemblages from the Ediacaran is the Avalon Assemblage, which mainly features mysterious fractal frond-like life forms known as rangeomorphs, who lived in the deep seas. The ancestors of the rangeomorphs may be evidenced by the earlier Protoarenicola, Pararenicola, and Sinosabellidites, which were previously interpreted as "metazoans" (the group containing animals), which date from around 800 mya. The worm-like Parmia dates from the beginning of the Neoproterozoic and may represent an even earlier ancestor of the group. The precise affinities of the rangeomorphs, as well as other participants in the Ediacaran biota, remain obscure, with some suggestion that they belong to a separate kingdom with the proposed name Vendobionta.
Later evidence, particularly from the White Sea Assemblage, suggests that some Ediacarans were capable of movement, such as Dickinsonia, possibly the earliest known animal, and the trilobite-like Spriggina. Other animals burrowed through the surface of the seabed, developing the first sensory organs over time, and eventually leading to the diminution and disappearance of the microbial mats which supported the other life forms, which may have contributed to their extinction and replacement during the course of the succeeding Cambrian period, though evidence also suggests that an anoxic event, similar to that at the beginning of the "Boring Billion," also occurred at the close of the period. One Ediacaran which survived into the early Cambrian was Cloudina and its relatives, which represent the earliest known possessors of mineralised skeleton-like structures, suggesting they were early reef-builders, along with the Calcimicrobes which are first evidenced near the beginning of the Proterozoic.
The most pertinent to the question of human origins is without doubt Kimberella, a genus which is found in the Ediacara Hills of Australia and close to the White Sea in the Russian Federation. Kimberella lived around 558-555 mya and represents the earliest animal to unambiguously exhibit bilateral symmetry (thus a member of the clade Bilateria). Slightly earlier, at the end of the Marinoan glaciation, is Vernanimalcula guizhouena an acritarch which may be bilateralian but this remains highly controversial.
More specifically, Kimberella may represent an early protostome (animals with a simple "mouth" structure), suggesting that the deuterostome lineage, which includes ourselves, all chordates, hemichordates and starfish, arose some time earlier: indeed, an early form of echinoderm (starfish-like animal) may be read into the fossil Arkarua, which shows the five-way symmetry familiar among these organisms today.
ALIVE! 2: THE RISE OF ANIMALS
THE CAMBRIAN EXPLOSION
After the major extinction event at the end of the Proterozoic Eon, during which much of the odd life which flourished through the Ediacaran Period died out, probably as a result of a huge drop in the amount of oxygen available in the ocean, the Cambrian explosion saw the development of most of the modern animal phyla which remain today. Among them were the Archaeocyatha, the first known reef-building animals, which appeared about 525 million years ago and lasted until the end of the Cambrian, and the Anomalocaridids, a group of shrimp-like animals which came to fill a wide variety of ecological niches, including that of apex predator (the earliest known example), and which grew to a length of about seven feet. One of the most iconic groups of fossil animals, the Trilobites, also appeared around the same time as the Archaeocyatha, their exoskeleta suggestive of further development of predator-prey relationships. Whilst the vast majority of animal life remained within the seas, some centipede-like animals were spending time on land, leaving fossilised footprints dating from around 530 million years ago.
The Cambrian also saw the coming and going of a whole host of weird and wonderful animals: these include the probable early filter feeder Aegirocassis benmoulai and the bizarre five-eyed Opabinia regalis - both probably relatives of the Anomalocaridids - as well as the spiny worm-like Hallucigenia. Spines also appear on the mollusc-like Wiwaxia.
Spines of a different sort were also beginning to develop: a structure known as a notochord is diagnostic of a group of deuterostomes known as chordates, represented by specimens such as Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis, known from the Maotianshan Shales (c.525 mya), as well as the slightly later Pikaia gracilens from the Burgess Shale (c.508 mya). The conodonts - important fossils in paleontology - also arose during the Cambrian. These evolved from the deuterostomes of the earlier Cambrian, represented by Saccorhytus coronarius (c.540 mya) and the Vetulicolia, and eventually gave rise to the Vertebrata - including us.
THE CAMBRIAN SUBSTRATE REVOLUTION
One major feature of Cambrian organisms was their increased use of burrowing as a means to escape predators, ambush prey and also for anchoring. Deeper burrows were needed for the latter purpose, as the microbial mats which typified Precambrian seabeds had been decimated by Ediacaran grazers. This in turn led to the development of a loose upper substrate rich in oxygen above a layered, sulphidic lower substrate.
This Cambrian substrate revolution (also known as the agronomic revolution) led to the extinction or adaption of animals reliant upon the microbial mats and contributed to the evolutionary radiation which occurred during this time.
THE GREAT ORDOVICIAN BIODIVERSIFICATION EVENT
The end of the Cambrian is marked by a series of minor extinction events, followed by another radiation of life, the Great Ordovician biodiversification event (GOBE), during which many of the classes of modern animals appeared to fill out the phyla which developed during the Cambrian. The GOBE was at one point thought to have resulted from the Ordovician meteor event (around 467.5 million years ago), though this is no longer the case. Among the animals which appeared during this time were the Eurypterids, better known as the "sea scorpions," whilst the Nautiloids, a group of molluscs represented today by the Nautilus, became common. Early forms include the Orthocerida, named for Orthoceras regulare, which prowled the oceans about 488 mya.
The Nautiloids were joined by the subclass Ammonoidea during the Devonian. This iconic fossil taxon, better known as the ammonites, were closely related to the ancestors of the modern-day groups of octopuses, squid and cuttlefish, and they survived until the end of the Mesozoic Era. The Belemnoidea lived throughout the same time frame as the ammonites and bore a closer resemblance to squid, and especially cuttlefish. One possible member of this group was the Jeletzkya douglassae from some 320 mya, which is considered a member of the crown-group of the squid. The Belemnoidea also includes the belemnites, which lived from the Triassic to the end of the Cretaceous.
The Ordovician also furnishes the first potential evidence of more complex land plants, known as polysporangiates, in the form of fossilised spores. Indeed, one potential contributor to the Ordovician-Silurian extinction event, a major dying-off which marked the end of this era, is the action of the primitive ancestors of mosses absorbing carbon dioxide and breaking down rocks, releasing phosphorus and iron into the sea, which caused algal blooms and eventually leading to polar glaciation. Other potential causes include a gamma ray burst from a hypernova.
SOMETHING FISHY...
Placoderms, armoured fish, first arose during the Silurian and became the earliest major group of vertebrates during the Devonian, supplying the first apex predators from among our lineage: the fearsome Dunkleosteous, a six-metre-long bone-plated leviathan which prowled the Late Devonian oceans. Another group which are first evidenced during the Late Silurian are the ancestors of modern sharks. By the Devonian, the early sharks were assuming forms weird and wonderful, like Stethacanthus (the "ironing board" shark), becoming even stranger during the Carboniferous with Helicoprion and its relatives, sharks boasting a singular "tooth-whorl."
Bony fish too emerged during this time, forming two groups: the Actinopterygii ("ray-finned fish") and Sarcopterygii ("lobe-finned fish"), likely evolving from one lineage of Placoderms: Entelognathus features a jaw - a defining feature of fish and tetrapods which evolved from gill arches during the Silurian - which resembles those of members of these groups. The former group includes most of the fish species known today. The earliest known member of this group was thought to be Andreolepis hedei, which lived around 420 million years ago in the Late Silurian. The Sarcopterygii includes the famous "Lazarus taxon" the Coelacanth, as well as the lungfish and, perhaps most notably, the ancestors of amphibians, reptiles, birds and mammals. This group's fossil record begins with Guiyu oneiros from about 419 million years ago. By 375 million years ago, one member of this group, Tiktaalik roseae, was possibly making its way from the sea onto land.
IF A TREE FALLS...
The Silurian saw further evolution among land plants, with the development of vascular tissue: Cooksonia, a genus which first appeared around 430 mya, and Baragwanathia from about five years later, represent early vascular plants. Asteroxylon and Drepanophycus, relatives of Baragwanathia, appear early in the subsequent Devonian, providing a rich habitat for the arthropod pioneers on land.
These plants initially had to play second fiddle to fungi, with Prototaxites, a gigantic tree-like fungus which lived about the same time as Cooksonia, reaching heights of 8 metres. Another probably early fungus is Tortotubus, as well as the dubious Ornatifilum. Furthermore, the mid-Devonian Spongiophyton is potentially a lichen. The earliest plant to achieve a similar grandeur about which we know was Wattieza, a relative of modern ferns and horsetails which lived around 385 mya in the Mid-Devonian. Archaeopteris, related to the ancestors of the spermatophytes (seed-bearing plants), straddled the Devonian-Carbonaceous boundary, and during this latter period, the atmosphere, vastly richer in oxygen than today, furnished an opportunity for massive growth. A group of lycopsids (clubmosses), the Lepidodendrales, took full advantage.
The Devonian expansion of plants on land included Runcaria heinzelinii, a transitional fossil precursor of the seed plants which developed throughout the remainder of the Mesozoic, giving rise to many of the plants we know today.
GROWING LEGS
The Sarcopterygii diverged during the early Devonian, with the ancestors of the coelacanths evolving separately from the rhipidistians. The latter group includes the Dipnoi or lungfish, as well as the clade Tetrapodomorpha.
The latter - which includes the aforementioned Tiktaalik - first emerges about 410 mya, the earliest known representatives being Tungsenia paradoxa and Kenichthys campbelli. Tinirau clackae lived around 387 mya, and shares a series of features with the tetrapods. Panderichthys rhombolepis (380 mya) and Tiktaalik represent further links in the evolutionary chain, which would eventually yield creatures akin to Ichthyostega, an amimal which would have borne a closer resemblance to the earliest true "amphibians" than to its fish ancestors. Ichthyostega dwelt in swampy terrain about 360 mya.
By the early Carboniferous, this lineage had produced basal tetrapods belonging to a series of families: the Whatcheeriidae, which includes the genera Watcheeria and Pederpes; as well as Crassigyrinus.
BUG OUT!
The end of the Devonian is followed by Romer's gap, a 15-million-year-long period which has yielded few fossils. After this hiatus, the fossil record reveals that the fish-like relatives of Tiktaalik and Ichthyostega were replaced by more advanced amphibians known as the Temnospondyls. The Carboniferous environment was typified by gigantic lycopsid trees, such as Lepidodendron and Sigillaria, horsetails, ferns and the conifer-like Cordaitales. Also present in the Carboniferous forests were the Pteridospermatophyta ("seed ferns"), an early type of spermatophyte which survived into the Cretaceous. By the Permian, the first Cycads - the most primitive of the modern conifers - had evolved, and were soon joined by the early Ginkgoales. Atmospheric oxygen was around 160% higher than today, enabling land invertebrates to grow to gigantic proportions. Famous members of these groups are the eurypterid Megarachne, the Late Carboniferous dragonfly Meganeura and an eight-foot long millipede, Arthropleura.
The Carboniferous rainforest collapse (CRC) occurred around 305 million years ago, changing the composition and size of the coal-forming forests and leading to the demise of many species. By the end of the Carboniferous, fungi had evolved the ability to break down dead trees, thus bringing to an end this period of fossil fuel production.
THE EGG, LONG BEFORE THE CHICKEN
Towards the end of the Carboniferous, some early tetrapods developed the ability to lay eggs with a mineralised outer layer, i.e. a shell. This enabled them to remain on land permanently, rather than having to find water in which to lay spawn as their amphibian relatives did (and, indeed, continue to do). These early reptilians, known as Amniotes, soon branched into two distinct groups.
The first of these are the synapsids, which are also known as stem mammals, as it is from this clade that the mammals arose. These proved to be the dominant group during the Permian and into the Triassic, before being eclipsed by the archosaurs. The synapsids had one temporal opening and are generally divided into two groups. Of these, the pelycosaurs include the fearsome Early Permian predator Dimetrodon, one of a number of members of the group to possess a sail-like appendage on its back, which may be interpreted as an early form of thermoregulation. These eventually gave way to the more mammal-like therapsids, a group including the Gorgonopsids and Cynodonts.
A GREAT DYING AND A LYSTROSAURUS INTERLUDE
The transition between the Paleozoic and Mesozoic is marked by the most attritional of the Phanerozoic mass extinctions, the Permian-Triassic extinction event or "Great Dying." This holocaust was likely caused by massive flood volcanos, evidenced today by the Siberian traps, a large igneous province covering around 1 million mi3 of the modern Russian Federation.
The Cynodonts were, along with the Theriocephalia and Dicynodonts, one of the groups who survived into the Triassic, and this group eventually gave rise to mammals. The non-mammalian Cynodonts survived - depending on classification - as late as the Miocene, some 17.5 million years ago, in the form of the Gondwanatheria. Of the other groups, the Dicynodont Lystrosaurus was the dominant land animal in the wake of Great Dying. The largest known Dicynodont is also the latest, the elephant-sized herbivore Lisowicia.
The Mesozoic, however, came to be dominated by another group, the diapsids, more particularly the archosaurs. The Archosauromorphs first emerged in the Permian, around 260 mya, and the archosaurs radiated in the wake of the extinction, replacing the synapsids in many ecological niches. These include the include the Phytosaurs and Rauisuchids (the ancestors of today's crocodilians), members of the lineage pseudosuchia, as well as the Avemetatarsalia, the ancestors of the iconic dinosaurs (which includes our modern feathered friends) and flying pterosaurs.
The dinosaurs emerge around 243-233 mya, and are represented in the Triassic by Nyasasaurus, the early theropods Eoraptor and Coelophysis and the Herrerasaurids. Other Triassic reptiles include the mysterious Longisquama insignis, which bore a series of feather-like structures along its back, while the Thalattosaurs, Hupehsuchia and the ancestors of the dolphin-like Icthyosaurs became increasingly better-adapted to exploiting an aquatic environment, and the Kuehneosaurids exploited the aerial niche formerly inhabited by Coelurosauravus. Also present were the seal-like Nothosaurs, ancestors to the Pliosaurs and Plesiosaurs of the age of the dinosaurs.
FUR AND FEATHERS
The Triassic-Jurassic extinction event brought to a close the domination of the pseudosuchia, as well as causing the temnospondyl amphibians to go into terminal decline. The Jurassic was to be the first period of the age of the dinosaurs, with this group, along with the pterosaurs, representing the largest terrestrial creatures until the end of the Cretaceous, when the mighty Tyrannosaurus rex, taking a brief moment out of a titanic clash with a Triceratops or Ankylosaurus, roared impotently in the foreground as the Chicxulub impactor approached the earth to carry out its work of devastation 66 million years ago.
Among the dinosaurs were the mighty sauropods (descendants of animals similar to the Triassic Plateosaurus) and the diverse ornithischians, though it was a series of fearsome theropod groups which would fill the role of apex predator: Ceratosaurus, Megalosaurus, Allosaurus and the enormous Spinosaurus were among them. Another group, which were prevalent during the Cretaceous, were the dromeosaurs (perhaps better known as "raptors"), an unfriendly bunch of feathered carnivores with shearing claws. Their relatives included the troodontids, probably the most intelligent non-avian dinosaur group, and Archaeopteryx, a considerably earlier animal (dating from around 150 mya) which shows an even greater number of similarities with the only dinosaur group to make it above the K-T boundary: the birds. By the end of the Cretaceous, this new Avian lineage had produced several now-extinct varieties of primitive birds. Among these were: the Confuciusornithidae, the first birds with a pygostyle (a "parson's nose") rather than a more reptilian tail, during the early Cretaceous; the Enantiornithes, of which some 80 species are known; and species adapted to life in and around the seas, such as Ichthyornis and Hesperornis. All of these groups retained to some extent claws on their wings and teeth. More modern birds were represented by, for example, the duck-like late Cretaceous Vegavis iaai, the Paleogene relatives of which include the fearsome flightless bird Gastornis.
The cynodonts were also evolving throughout this period, though by now restricted to marginal lifestyles and small sizes. Already by the late Triassic, these were taking on forms which would be commonplace among later mammals. Early representatives of these mammaliaformes include Adelobasileus, Tikitherium (c.225 mya), Morganacudon (c.205 mya) and Megazostrodon (c.200 mya). Also dated to about this time, according to genetic studies, the Yinotheria, ancestors of the monotremes (surviving today in the form of the platypus and echidnas), branched off from the other mammals. Their earliest surviving members are the Shuotheriidae, around 165 mya.
These mammaliaformes would continue to develop into a number of distinct groups of mammals in the Jurassic and beyond. These include: the Allotheria (ancestors of the long-lived Multituberculata, which survived until 17.5 mya) potentially dates from the Triassic, if the order Haramiyida are included; the Triconodontidae (190-70 mya); the Docodonta (including Castorocauda, a semi-aquatic, beaver-like animal living about 165 mya); the gliding mammal Volaticotherium antiquum; the Dryolestoidea, which survived from the Jurassic into the Miocene; and the Cretaceous order Symmetrodonta. The Metatheria, the clade which includes modern marsupials, is first evidenced by Sinodelphys szalayi, from the Yixian Formation in China.
Among these is also the earliest known eutherian (the group encompassing placental mammals), Juramaia sinensis, which was found in China in rocks dating to around 160 mya. It would be Juramaia's successors which would, after the Chicxulub event, inherit the earth.
Also present in the Early Cretaceous was a plant, Archaefructus liaoningensis, which represents the earliest angiosperm (or flowering plant) known from the fossil record.
ALIVE! 3: THE RISE OF MAMMALS
THE MEEK INHERIT THE EARTH
The shuffling of the non-avian dinosaurs off this mortal coil led to situations vacant in a wide range of ecological niches. The early Paleocene saw mammals, birds (including the successful and long-lived flightless turkey from hell Gastornis) and crocodilians, among others, stepping up to the plate in a competition to secure the best of these. Mammals - which were by no means exempt from the effects of the extinction (metatherians in particular being hard hit) - began small, being at this stage a disparate bunch of rodent-like animals, though, once this opportunity presented itself, they began to diversify in a series of weird and wonderful ways.
The types of placental mammals which are attested in known Paleocene geological levels remain mysterious, with their exact relationships to the orders known from the Eocene to today, not to mention their affinities to one another, still the topic of a good deal of debate among paleontologists. What is known is that, among their number, remains of animals closely related to the ancestors of a number of later orders, including the Primates, are to be found.
Many of the remainder have been placed in the convenient "wastebasket taxon" Condylarthra: these early ungulates (hoofed mammals) are a motley bunch, including the Arctocyonidae (which includes the omnivore Arctocyon, which reached sizes up to that of a small bear during the Paleocene, as well as the arboreal Chiracus), herbivores such as Phenacodus and others such as the Periptychidae. Perhaps related to the Arctocyonidae is the order Mesonychia, encompassing a host of hoofed carnivores dating from the Paleocene into the Oligocene, the largest of which during the Paleocene was the early form Ankalagon saurognathus. The Creodonta, which largely replaced the Mesonychids as top proto-dogs, were also evolving during this period, while the ancestors of the Carnivora which would take their place are also known: these are small, probably tree-dwelling animals grouped in the family Miacidae. Early herbivores include the Pantodonta, Tillodonta and Taeniodonta, with a pantodont, Titanoides, reaching lengths of up to 10 feet during the Late Paleocene.
Also present in the Paleocene was another order, the Plesiadapiformes. These animals, probably descended from the K-Pg extinction survivor Purgatorius, were closely related to the ancestors of today's Primates. Members of this order include Micromys, Carpolestes and Plesiadapis, as well as a possible glider in Phenacolemur.
By the early Eocene, a number of other Primate groups appear in the fossil record, including the Adapidae (relatives of the lemurs) and Omomyidae (with affinities with the tarsiers). The earliest monkey-like primates to appear are members of the family Eosimiidae, the earliest of which, Phileosimias, dates to around 45 mya. One possible member, Afrasia djijidae, appears in both Asia and Africa about 37 mya.
AN EOCENE EDEN AND A PROLONGED COLD SNAP
The boundary between the Paleocene and Eocene is marked by an increase in global temperature, culminating in the Paleocene–Eocene Thermal Maximum, around 55.5 mya.
While ocean life suffered (with 35-50% of benthic foraminifera disappearing), this gave an incredible opportunity on land, enabling plants and animals to spread out in the direction of the poles. Temperatures decreased steadily throughout the Eocene, however, leading to the eventual glaciation of Antarctica about 34 mya - a cold spell which still persists to this day.
WALKING WHALES AND DOG-SIZED HORSES
The early Eocene also provides evidence of the starting points for two well-understood evolutionary lineages: whales and horses. The former group emerged from a primitive family of Artiodactlys (even-toed ungulates, such as cattle, sheep, pigs and goats, as well as camels and hippos) called the Raoellidae, which includes the genus Indohyus (c.50-48 mya). Indohyus was a contemporary of Pakicetus inachus, which may have already begun to adapt to a semi-aquatic lifestyle. A few million years later, Ambulocetus natans was already fully aquatic and adapted to both fresh and salt water, though the animal retains large toes. Remingtonocetus (c.45-43 mya) marks the next phase in Cetacean evolution, followed by the Protoceticae, the best-known of which is probably Rodhocetus.
By about 40 mya, the Archaeocetes had produced two sub-families well-adapted to the seas: the Basilosaurinae and Dorudontinae. These were in turn closely affiliated with the Odontoceti (toothed whales) and Mysticeti (baleen whales) which still roam the seas today, as they have since the late Eocene.
Meanwhile, the primitive horse-like Eohippus gave rise to Orohippus, which evolved into Epihippus by about 47 mya. After the evolution of grasses and the subsequent conversion of much of the forest into prairies and grasslands as the Eocene gave way to the Oligocene, Mesohippus appeared (c.37 mya), followed in short order by Miohippus, the ancestor of the later equines which appeared in the Miocene.
HELL PIGS AND HORRIBLE HYAENODONS
The Eocene and Oligocene saw a great many fearsome and often bizarre creatures come and go, a number of which are related, however distantly, to animals still alive today. Foremost among this category are a number of large, probably omnivorous, species belonging to the Artiodactyls. The most iconic of these is without doubt Andrewsarchus mongoliensis, which is known from the Middle Eocene of Inner Mongolia. The skull of this animal is vast in comparison with mammals exploiting similar niches at other times, possibly indicating that Andrewsarchus was the largest mammalian predator of all time.
Andrewsarchus' precise affinities are unclear, though it is likely that the creature was related to the ancestors of the Cetaceans, as well as another group of large omnivores which first appear about 10 million years after Andrewsarchus' time. These are the entelodonts, perhaps better known by their byname of "hell (or terminator) pigs," a moniker created to reflect their morphological similarities to modern pigs and their relatives. The entelodonts survived into the Miocene and reached their largest size during the period around the Oligocene-Miocene boundary in the form of Daeodon shoshonensis. The skull of Daeodon indeed suggests that the animal was comparable in size to Andrewsarchus. Contemporary with Andrewsarchus was Mesonyx, which, along with the slightly earlier Pachyaena, were among the largest of the later Mesonychids.
By this time, however, these hoofed predators had to contend with the Creodonta, which were growing in size and increasingly competing with them. The creodonts belong to two families, the Oxyaenidae and Hyaenodonta. The latter are named for Hyaenodon, the most substantial of which was H. gigas, reaching lengths of three metres. The Oxyaenidae includes the likes of Patriofelis and the huge Sarkastodon mongoliensis, which reached a similar length to H. gigas, and lived about 35 mya.
BEAR-DOGS AND BONE CRUSHERS
Early carnivorans of the Eocene and Oligocene include the Nimravidae, members of the suborder Feliforma, which lived between around 40 to 7.25 mya. These are named for the Oligocene member Nimravus, and are sometimes given the nickname of "false sabre-toothed cats," due to their providing early evidence for this famous feline innovation. The Barbourofelidae, which arose during the Miocene, continued this trend, which eventually led to the fearsome Smilodon gracilis, a member of the Machairodontinae subfamily of the true cats or Felidae, a family first attested around the time of the Eocene-Oligocene boundary, in the form of Proailurus.
The Caniforma are represented by early members of the clades Amphicyonidae ("bear-dogs") and, to add to the confusion, the Hemicyonidae or Hemicyoninae ("dog-bears"), the latter of which are closely affiliated to modern bears in the family Ursidae, as well as the Hesperocyoninae, alongside their offshoots the Borophaginae ("bone-crushing dogs") and the early true Canidae, such as Cynodictis. Towards the end of the Oligocene, another couple of members of this order, Puijila darwini and the genus Enaliarctos, represent early steps in the evolution of modern pinnipeds, the clade encompassing seals, sea-lions and walruses.
FIRE-BEASTS AND THUNDERING TITANS
Meanwhile, a series of herbivore groups were also gaining in size and number. Early on, one particular group, the Dinocerata - whose best-known member is the Eocene Uintatherium of North America - were reaching large sizes. The earliest members of this group were Prodinoceras in Asia and the North American Probathyopsis, during the Late Paleocene, while later members include Eobasileus cornutus, which boasted an elaborate set of horns.
The affinities of the Dinocerata have long been unclear, though they now appear to have been relatives of Carodnia, a genus of early Eocene ungulates from South America. This continent, which was isolated prior to the creation of the Isthmus of Panama and subsequent Great American Interchange during the Pliocene, was home to a rather unique fauna, which included the metatherian Sparassodonta (a group which included another sabre-tooth, Thylacosmilus) and giant Xenarthra (relatives of modern sloths, armadillos and anteaters), was also home of a number of placental ungulate groups grouped together as Meridiungulata. These include: the Xenungulata (the order which includes Carodnia, which spans a period from about 58 to 48 mya);the probably-semi-aquatic Pyrotheria ("fire-beasts"), which died out during the Oligocene; the Astrapotheria; Litopterna; and Notoungulata.
Africa too had a fauna all of its own, the Afrotheria. This superorder, which is represented today by elephants, sirenians (the dugong and manatee), tenrecs, hyraxes and others, includes the Eocene-Oligocene Embrithopoda, which also boasted large horns, as well as the Desmostylia, an extinct group of sea dwellers which appeared in the Oligocene and survived until around 7.2 mya. The carnivorous Ptolemaiida also prowled the African landscape during the Oligocene.
SHUFFLING THE PACK: THE GRANDE COUPURE
As noted above, the transition from the Eocene to the Oligocene was marked by a significant radiation of the grasses, with an attendant changing of landscape from forest to plain. Concomitant with this was another evolutionary turnover, the Eocene–Oligocene extinction event or Grande Coupure ("great break"), which saw the extinction of many groups and their replacement by others. Paleontologist J.J. Hooker and his colleagues sum up the situation as follows: -
WHEN IS AN ELEPHANT NOT AN ELEPHANT?
The Neogene also saw a radiation among the Proboscidea, the group which contains the modern species of elephants, as well as the iconic mastodons and mammoths of the surprisingly-recent past. Early Elephantiformes include the Eocene and Oligocene Phiomia and Palaeomastodon, and it is this group which would fill out considerably during the Miocene and beyond. However, some members of groups which radiated earlier (the Plesielephantiformes) lived on, including the Deinotheriidae,whose latest representative, from the species Deinotherium bozasi, survived into the Pleistocene.
The Mammutidae developed from Oligocene animals such as Eozygodon and Losodokodon into the likes of Zygolophodon tapiroides with its vast tusks, and Mammut americanum, the American mastodon. Later offshoots from the surviving lineage include the Gomphotheriidae and the truly bizarre Amebelodontidae, as well as the relatives of the huge Stegodon. The remaining subfamily are the Elephantinae, which includes the genera Loxodonta (African elephants) and Elephas (Indian/Asian elephants), as well as the genus Mammuthus (the mammoths, not to be confused with the Mammutidae or mastodons). The best-known representatives of this genus are the Columbian (M. columbi) and - another icon - the wooly mammoth (M. primigenius), the latter of which survived on Wrangel Island in Arctic Russia until as recently as perhaps 300 BC.
THE VIEW FROM DOWN UNDER
Australia too was home to a variety of marsupial and other life then as now. These include the enormous Diprotodon and the "marsupial lion" Thylacoleo carnifex, alongside the titanic lizard Megalania (or Varanus) priscus - all of which survived to witness the first appearance of our own species on the continent - as well as a number of other, earlier creatures, such as large, carnivorous relatives of today's rat-kangaroos (Ekaltadeta and Propleopus), and the lineage of the recently-extinct Tasmanian tiger, Thylacinus cynocephalus. Less well-known are the tapir-like Palorchetidae.
FROM MONKEY TO APE
By the Oligocene, the ancestors of the Catarrhini (Old World monkeys and apes) and Platyrrhini (New World monkeys) of today had separated, each following its own path. In the case of the former, this led to Saadanius hijazensis, a close affiliate of the last common ancestor of both the Old World monkeys and our own lineage, the Hominoidea, which lived about 28 mya.
By the beginning of the Miocene, a number of genera belonging to the superfamily Hominoidea had arisen, including Ekembo, the Proconsulidae (such as Proconsul and Ugandapithecus), Dendropithecidae (including Dendropithecus, Micropithecus and Simiolus)and the Afropithecidae (such as Morotopithecus and Afropithecus). The last mutual ancestor shared by the Hominidae (great apes) and Hylobatidae (gibbons) lived around 20 mya during roughly this same period.
By about 15 mya, the African group (ancestral to ourselves, chimpanzees and gorillas) had diverged from the Asian ancestors of the orang utans, with the latter, the Ponginae well-represented in the fossil record by genera such as the 12 mya Sivapithecus and the famous Gigantopithecus - whose largest member, G. blacki, reached a height of up to ten feet and survived up until only about 100,000 years ago.
The ancestors of the Homininae may well have spent a good deal of time in Europe, where a number of Miocene fossils are attested, starting with Pierolapithecus catalaunicus, which dwelt in Catalonia some 13 mya. An early tribe of great apes, the Dryopithecini, includes a number of genera dating from about 10 mya, including Nakalipithecus from Africa and the Eurasian Dryopithecus and possibly Oreopithecus. Also present in Europe between 8.7 and 7.4 mya was the genus Ouranopithecus, the precise affiliations of which are as yet unclear, while Samburupithecus lived in Africa about 9.5 mya.
The Gorillini make their first appearance around 8 mya, in the form of Chororapithecus abyssinicus from the Afar Depression of Ethiopia, while the last common ancestor shared by ourselves and the genus Pan - which includes the common chimpanzee and bonobo - lived perhaps some 7.5 mya. The species Graecopithecus freybergi, which inhabited Greece around 7.2 mya, might fit the bill, or else it could be the oldest human ancestor after the separation. However, at this point, Africa takes centre stage in the story of human evolution.
BECOMING HUMAN
FROM APE TO MAN
Some time around 7.5 mya, the evolutionary lineages which would lead to humans and the chimpanzees diverged. 500,000 years later, a new species of Hominin was ekeing out a living in what is today the Djurab Desert in the African country of Chad, and perhaps spending some periods of time walking on two legs. Some remains of a member of this species, given the name Toumaï, were discovered in the early part of the third millennium, and were assigned the binomial name Sahelanthropus tchadensis. Slightly earlier, the first of a series of fossils belonging to the species Orrorin turgensis were uncovered in Kenya. Orrorin shares a number of features with us which differ from later human ancestors, particularly in terms of its dentition (the make-up of its teeth), and Orrorin may also have spent time walking upright.
One possible reason for this novel means of locomotion is an unusually high numbers of wildfires between 7 and 8 mya - potentially caused by a supernova! - which transformed the swathes of forest which covered parts of Africa into something resembling the savannas and grasslands we see today. Another such event, between 2 and 3 mya, likely sealed the deal. Walking upright would have been an advantage in that it allowed these small apes to peer over the heads of the grasses to get advanced warning about predators lurking in the area, such as the sabre-toothed cat Dinofelis, as well as staking out potential meals of their own from afar.
By about 5.5 mya, a new genus, Ardipithecus developed, and is represented by two known species: the Miocene A. kadabba and Pliocene A. ramidus.
By about 4.2 mya, the genus Australopithecus had emerged in eastern and southern Africa, being represented by A. anamensis. A. anamensis soon transitioned into A. afarensis, which lived for a period of about a million years from 3.9 mya. Australopithecus is more suited to bipedal walking than Ardipithecus and its predecessors, which appear to have spent more time in the trees and less on the ground. Two related species, A. bahrelghazali and A. deyiremeda, appeared about 3.5 mya, with A. africanus - the first member of the genus to be discovered - evolving shortly thereafter. Also present at this time was a new genus, represented by Kenyanthropus platyops, and it is this species which may well have initiated tool shaping among the hominins: the earliest-known lithic industry is the Lomekwian, which dates from about 3.3 mya, and was uncovered at the Lomekwi 3 site in Kenya, in close proximity (both physically and temporally) to strata bearing Kenyanthropus fossils. The Lomekwian industry precedes the Oldowan or Mode I - from which the later stone tool industries derive - by some 700,000 years.
The Oldowan industry may well have been developed by a later Australopithecus species, A. garhi, which emerged around the same time, as the Pliocene epoch drew to a close. A. garhi was contemporary with the last 500,000 years or so of the lifespan of A. africanus, with the last species assigned to the genus Australopithecus, the more advanced and Homo-like A. sediba appearing around 2 mya.
THE NUTCRACKER SUITE
Another genus, Paranthropus, appeared at about the same time as the Oldowan technology, and isrepresented in the fossil record by three known species. Paranthropus is characterised by a more robust skull shape than that of the Austalopithecines, bearing a sagittal crest across the top of its head similar to that of a gorilla, and pronounced attachments for strong muscles capable of chewing tough vegetation. Paranthropus also has a cranial capacity similar to that of the earlier Australopithecines, which enables us to conclude that Paranthropus is a more specialised hominin, diverging markedly from the lineage leading to us.
The earliest member of the genus is P. aethiopicus, which dates between 2.7 and 2.5 mya, and likely gave rise to P. boisei, which appears shortly after the latest known member of P. aethiopicus. P. boisei survived until shortly before 1 mya, and, for much of its lifespan, it was contemporary with the third species, P. robustus. P. robustus is particularly interesting in that it shared more features with A. africanus than its fellow Paranthropus species. These affinities might suggest that Paranthropus was paraphyletic.
THE HANDY MAN
The earliest-known species assigned to our own genus, Homo, was also living during this time period. H. habilis first emerged early in the Pleistocene, about 2.3 mya, and probably died out about 1.5 mya. The transitional nature of this species - seemingly located somewhere between Australopithecus and H. erectus - has led to continuing debate among paleoanthropologists about the validity of its membership of Homo. H. habilis did, however, have a slight increase in cranial capacity over the Australopithecines, up to about half of that of modern humans, with the contemporary A. sediba only packing a third of the amount of grey matter we boast.
OUT OF AFRICA
Subsequently, two more species of Homo appear: H. ergaster and H. erectus. Of these, the former is a chronospecies which dwelt in Africa between 1.9 and 1.4 mya. H. ergaster can be seen as an African variety of H. erectus, the first human species to leave Africa.
H. erectus, of which there are any number of proposed subspecies, is associated with the transition from the Oldowan to the Acheulean (Mode II) stone tool industry, and its dispersal into western Europe and southern Asia. The earliest known representatives outside of Africa have been found in the Dmanisi cave in Georgia, and date from about 1.8 mya. During the course of the next 500,000 years, the species made its way into modern China, travelling as far as Java in Indonesia by about 1.4 mya. The latter subspecies, H. e. palaeojavanicus, is sometimes known as Meganthropus on account of the large jaw fragment discovered in 1941 by Gustav von Koenigswald, which has led to various unfounded theories about this species supposedly being of gigantic proportions.
HOBBIT ERECTUS?
H. erectus is attested throughout China and Indonesia for well over a million years, yielding iconic fossils such as Java Man (c.1-0.7 mya) and Peking Man (c.0.7 mya), surviving until perhaps as recently as 143,000 ya, in the form of Ngandong (Solo Man), who dwelt along the Solo River on Java. Meanwhile, in China, Nanjing Man survived until perhaps about 250,000 ya, with his contemporary Yuanmou Man pehaps dating from about 500,000 ya. H. erectus also persisted surprisingly late in Europe, depending on the interpretation of human remains from Bilzingsleben, Germany (which date from some 370,000 ya and show possible signs of ritual activity around disposal of the dead). Tautavel Man, discovered in the Caune de l'Arago near Tautavel, France, dates from about 450,000 ya.
The most recent in date - and remarkable - finds related to this early human dispersal are probably the small hominid species found on islands in the western Pacific Ocean. Thus far, two new species of Homo have been described. H. floresiensis, otherwise known by its Tolkien-derived nickname "hobbit," lived on the Indonesian island of Flores up until about 50,000 ya, while H. luzonensis, a taxon described as recently as 2019, inhabited Luzon in the Philippines at around the same time. These species represent descendants of H. erectus displaying "island dwarfism" - and, as such, it is worth noting that, close to the cave of Liang Bua, in which the remains of H. florensis were discovered, live a group of people called the Rampasasa, inhabitants of Waemulu village, who also display traits island dwarfism. There is, however, no genetic connection between the Rampasasa and the hobbit: this is convergent evolution in action.
These small hominins were not the latest enigmatic humans though: about 14,500 to 11,500 years ago, the Maludong ("Red Deer") and Longlin Caves in southern China were inhabited my a mysterious, archaic-looking group with notably large cheekbones. Known as the Red Deer Cave people, this group have been variously explained as archaic humans, robust early modern humans or a stable Denisovan-H. sapiens hybrid population.
BIG BRAIN, SMALL BRAIN
Another group of fossils dating from relatively recently (probably about 250,000 ya) indicates that early members of H. sapiens dwelt alongside other hominins with very different morphological traits in Africa. This new species, H. naledi, is found in the Rising Star Cave in Gauteng province, South Africa. H. nadeli bore a number of traits shared with the much earlier H. ergaster and even Australopithecus, in combination with other more derived features. The brain of H. naledi was small, not much more than half that of a typical H. erectus, and the average male member of the species stood about 5 feet in height, slightly smaller than other contemporary members of Homo. Nonetheless, H. naledi's brain was structurally similar to modern human brains, and the species may well have used the cave system as a site for funerary ritual.
THE ASCENT OF MAN
Evidence for an archaic human presence has been uncovered in Spain, France and England dating to around 1 mya. These people are members of a proposed species, H. antecessor, though scientists generally ascribe them to H. erectus (as a European variety), or regard them as an earlier form of a species which is likely the immediate forebear of our own: H. heidelbergensis. H. heidelbergensis is known from African and European contexts and, in its developed form, is regarded as having lived between about 750,000 and 250,000 ya. H. heidelbergensis evolved from H. erectus, and the European branch of the species gave rise to the famous Neanderthals (H. neanderthalensis).
Meanwhile, in Africa - where Lee Berger claimed to have uncovered remains of a number of H. heidelbergensis over 7 feet in height (while these assertions have not as yet been published in a peer-reviewed journal that I know of, Berger remains one of the paramount palaeoanthropologists of the early 21st century and, as such, cannot be quite so easily dismissed as many other people claiming to have discovered giants of some description) - the picture is muddied somewhat by the presence of another proposed species, H. rhodesiensis, which may instead by an African form of H. heidelbergensis. Unfortunately, there is at present something of a dearth of African fossils dating from 400,000 to 260,000 ya, so the precise picture of the emergence of H. sapiens from these earlier forms is, as yet, unclear. It must, however, be stated that distinctions between species are rather arbitrary: there was no single point at which, for example, two members of H. erectus became the proud parents of the first H. heidelbergensis. Additionally, the earliest member of H. sapiens identified to date, discovered at Jebel Irhoud in Morocco, dates from 300,000 ya, which suggests a much earlier radiation of H. sapiens than was previously assumed. Furthermore, the Gawis cranium, which could well be even earlier, is regarded as transitional between H. erectus and H. sapiens. Add to this mix the remains of Herto Man, found in Ethiopia and dating to about 160,000 ya, which has been ascribed as a subspecies of our own, H. s. idaltu.
OUT OF AFRICA 2: THE FIRST WAVE
Already by this period, H. sapiens had begun to migrate out of Africa. By 215,000 ya, this early group of explorers - who have seemingly left no trace in our genetic makeup - had made it as far as Greece, where their remains were discovered in 1978. Dali Man, who was also possibly a member of our species, died in China at roughly the same time - and could be taken as indicative of the multiregional hypothesis favoured by some scientists, particularly in China, which posits that modern humans, migrating out of Africa, met and interbred with other Homo species in Europe and Asia.
NEANDERTHALS, DENISOVANS AND US
Europe and western Asia was, by this stage, largely the domain of the Neanderthals. This species boasted the highest cranial capacity of any Homo species (albeit evidence of earlier puberty among Neanderthals than H. sapiens likely means less time to develop quality to go with the quantity), and lived in the inhospitable northern climes of the last Ice Ages, to which they were well-adapted, hunting large game and living off a diet which was almost exclusively made of meat. The Neanderthals were, however, not the grunting cavemen imagined of old: instead, they were artists - the earliest European cave paintings, from about 65,000 ya, were almost certainly the work of the Neanderthals. They likely went extinct around 43,000 ya, mainly through interbreeding with modern humans, bequeathing up to 4% of the DNA of modern non-African humans. Both modern humans and the Neanderthals also interbred with another, highly enigmatic, species known as the Denisovans. This group are represented in the Altai mountains by fragmentary remains, though one specimen, named Denny, is the fossil of a young woman who lived around 90,000 ya and who seems to have been a Neanderthal-Denisovan hybrid. The Denisovans also left a genetic legacy among the Melanesians and indigenous Australians.
