Although the timeline of early life is subject to differing interpretations, the current thinking is that it began around 3.8 billion years ago. The simple cells split into two early groups, the bacteria and the archaea, although which one was first is still undetermined. For over a billion and a half years life stayed that way, but around two billion years ago the first complex single cells evolved, the eukaryotes, with nuclei and other internal organelles. About 1.5 billion years ago the eukaryotes split in three groups: animals, plants, and fungi. Yet another long period of evolutionary stasis occurred until some time around 900 million years ago (MYA), multicellular life first appeared. According to Nick Lane, in his book Life Ascending, it took this long because this was a one-time, extraordinary, one in a trillion event where two cells merged without one consuming the other, and instead entered into a symbiotic relationship which vastly increased the new cell’s ability to survive and thrive. “The eukaryotic cell only evolved once because the union of two prokaryotes, in which one gains entry to another, is truly a rare event, a genuinely fateful encounter. All that we hold dear in this life, all the marvels of our world, stem from a single event that embodied both chance and necessity.”
The advantages of multicellularity gave evolution the opportunity to diversify plant and animal forms into the available environmental niches, and for another three hundred million years the successful experiments, such as bilateral symmetry, spread and flourished in the oceans. These early forms were tiny and soft-bodied, and did not leave fossils, although in some cases their presence could be inferred from the traces left behind, such as tracks and burrows. The first, disputed, fossils date from about 580 MYA but by 565 MYA there is clear evidence of animals having evolved the ability to move under their own power. The Cambrian Explosion started around 540 MYA and lasted for about 20 million years. By that time the great phyla of life had already come into existence. Evolution was exploiting all the available niches, and the predator/prey arms race was in full swing, with teeth and claws pitted against speed and shells.
This book concentrates on those early, faint traces of life, citing facts where facts are available, and making inferences where there is no direct evidence. It is also a book about what paleontologists really do, as opposed to what many people think they do. It all sounds exciting and romantic until you realize that the field work often involves sitting in the dirt all day long brushing away soil centimetre by centimetre with a trowel, and then back to the laboratory to spend years laboriously extracting fossils from the surrounding matrix using tiny dental drills.
Charles Darwin took decades after this voyage on the Beagle to review the evidence he had collected, and to amass new evidence from his studies of barnacles and discussions with dog and pigeon breeders. He slowly, almost reluctantly, came to the conclusion that evolution is real and that natural selection is the engine which drives it. When he published Origin of Species in 1859 he made a strong case, but there were still unanswered questions, which his opponents were quick to cite as evidence of the weakness of his arguments. These questions included how evolution could account for altruism such as in ant and bee communities, or how the early stages of complex structures, like as an eye or a wing, could have provided survival advantages great enough to allow them to be passed along to their descendants. He was also concerned that life seemed to have just popped into existence in the Cambrian era, since the earlier rocks looked to be barren of life. In time, of course, all of these objections have been elegantly answered, and in the case of early life the evidence was there all along, but it was too tiny to see.
Dr. Brasier has spent his professional life examining the minute traces of early living things, and has traveled the world in search of answers. He recounts digs in such far flung places as Siberia, Outer Mongolia, China, and the Australian Outback, as well as Scotland, Newfoundland, and the Caribbean. The description of each place adds a piece to the puzzle, and since knowledge is cumulative, as the book progresses the reader comes to a better and more complete understanding of what was going on in those early millions of years before evolution lit the fuse of change.
The book looks at evolution from the perspective of environments rather than individual species. A species is the way it is because it has evolved to exploit a specific niche, and its ability to further evolve is constrained by the other species around it. When the environments are stable the plants and animals reach the point where they are almost perfectly adapted, and fossils show that they can remain unchanged for millions of years. When something disturbs the environment, however, such as the arrival of a new predator, or a heating or cooling event that wipes out some of the other species, the door is open for evolution to try new shapes and behaviors. All this can happen quickly, too. In Richard Dawkins’s The Greatest Show on Earth he recounts experiments with zebra fish which showed that, in the absence of predators, within a few generations males evolved bright colors to attract females, but with predators present they quickly developed duller spotted patterns to mimic the color of the stream bottom.
Although written in an easy to read style, this book is for people who already know something about evolution and are interested in how it works and what it shows about how life changed and adapted in the eons before humankind. There is something satisfying about seeing the slow accumulation of knowledge, as one new fact supports the ones that came before and provides a starting point for further ideas. This is science well done: evidence-based, cautious about making bold pronouncements, careful to ensure that when the author presents an idea that is not fully supported by existing findings it is identified as plausible but conjectural. I enjoyed the book and learned some interesting things from it.