Image from Wikimedia Commons
Although evolution has likely been happening since the advent of life some 3 – 4 billion years ago, life remained relatively simple until 600 million years ago, which even in terms of geological time is an impressie length. Of course, life needed much of this time to evolve many features of modern cells that we take for granted. The first prokaryotes were, as far as we know, all chemoautotrophs, which, as you should remember, are organisms that utilize chemicals, mainly CO2 in this case, to oxidize materials which in turn allows them to store energy. Glycolosis became common about 3.8 billion years ago, which has remained the dominant form of energy absorption to this day; this was soon followed by photosynthesis about 3.5 BYA (billion years ago), which was the latest estimate for Archaea and Bacteria splitting on the evolutionary tree. The prokaryotes gradually refined their cellular processes until about 1.9 BYA, when eukaryotes diverged from Archae, which was likely due to endosymbiosis. Organisms again entered a period of relative evolutionary rest, until about 1.2 BYA, where simple multicellular organisms, little more than groups of cells in a clump, and sexual reproduction become present. Another period of evolutionary rest ensued for another half billion years. Around 850 million years ago, the planet went into an ice age, which likely slowed evolution’s already torpid pace to a crawl. The thawing of the ice, and the advent of an ozone layer, however, gave rise to the first large evolutionary radiation, known as the Avalon explosion.
Occurring some 600 million years ago, the Avalon explosion has little explanation behind it, although leading theories include the increase in oxygen content of the atmosphere due to photosynthesis, or the ozone layer blocking out harmful rays, although these should already have been nullified by the ocean, where all known life at this time lived. This explosion led to a bizarre period in the history of life known as the Ediacaran period. The life forms in this period were predominantly soft-bodied, and left behind few fossils for scientists to study. The fossils that have been discovered are quite enigmatic in and of themselves.
Image from Wikimedia Commons
There was a significant degree of diversity among these species, some were disc shaped, although they were not jellyfish. Some scientists believe these disc fossils were large groups of microbes on the ocean surface, others believe that they were scratch marks left by organisms anchoring themselves to the sea floor, and others maintain that they were simply disc shaped organisms. Perhaps the most important species living at that time was Charnia, which resembled a leaf. It would have anchored itself to the ocean floor, and filter fed. It is the first species discovered from the Ediacaran, and showed paleontologists that complex life had indeed existed before the Cambrian period. Some species, whose fossils resemble “mud-filled bags” exhibit strange morphological features not seen anywhere today. It resembled a centipede, but without any appendages of any sort, including eyes, mouth, legs, and pretty much anything else you would expect in an organism. It is speculated to have grown by adding identical segments of its body to itself laterally, imagine your finger growing by adding a new joint rather than the entire thing growing. Known as Pteridinium; it, like most Ediacaran organisms have no known descendants. The lack of any known relatives of these bizarre forms of life, of which only a few were mentioned above, has led to speculation that these species are in fact unrelated to today’s life, and that life today has its roots in a completely different organism becoming multicellular. These organisms were rendered completely extinct by the mid-Cambrian, due to the fact that most forms of life in the Cambrian had hard shells, rendering them immune to the minute amount of predators present in the Ediacaran, and the increase in predatory and grazing organisms in the Cambrian led to the stationary filter feeders being completely wiped out.
The organisms in the Cambrian that led to the end of the Ediacaran organisms arose due to an event taking place some 540 million years ago known as the Cambrian explosion. Compared to the Avalon, the Cambrian explosion is well documented. It is speculated that factors ranging from an increase in global temperature, the advent of eyes creating a race for defenses against predators, an increase in oceanic calcium, and an alternative explanation of the end of Ediacaran life wherein all those organisms went extinct, and the Cambrian species radiated to take their place.
Life has existed for a very long time, but evolution went quite slowly for the first billion or so years. Multicellular life has evolved independently 49 different times (at best estimate) but only two of these times has it created a significant ecosystem. It was not until these ecosystems got to the point that their cellular processes could alter the environment around them that significant evolution could take place.