Written by Graham | Created: Friday 12th July 2019 @ 1036hrs | Revised: Thursday 1st October 2020 @ 0106hrs
The Hadean and Archean Eons
4.54-2.5 billion years ago.
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.
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: -
[...] it is possible - indeed, I believe, likely - that early DNA life on Earth might have come in a variety of forms, perhaps all slightly different from our now familiar DNA. If so, it is probable that separate kinds of DNA competed against each other. [...] It is hard to believe that our complex variety of DNA appeared fully formed, without a competing cohort of slightly different versions.
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."
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.
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.
The Paleoproterozoic Era
2.5-1.6 billion years ago.
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.
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.
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.
The Mesoproterozoic Era
1.6-1 billion years ago.
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 Neoproterozoic Era
1 billion-547 million years ago.
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.
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.
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 Parmiadates 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.