November 28, 2015 10:53:40 AM
Right at the beginning of The Story of Life in 25 Fossils: Tales of Intrepid Fossil Hunters and the Wonders of Evolution (Columbia University Press), paleontologist Donald R. Prothero shows Darwin's importance as a scientist. Oh, of course, Darwin had the brilliant insight about evolution, and had all sorts of admirable qualities as a scientist and as a human being. But one of his best traits was that he knew, and acknowledged publicly, where his theory was weak. He admitted that could not, when his On the Origin of Species was published in 1859, answer questions about transitional species, those links between ancestors and progeny showing how animals changed in their lines of descent. Prothero explains that when Darwin was writing, almost nothing was known of transitional fossils. Darwin's insights had to be made without their evidence. Now they are among the strongest confirmations of evolution, and Prothero here takes us through 25 examples to tell the stories about how the evidence was amassed and interpreted. There are millions of species he could have chosen, but these 25 show critical stages of evolution and also some of the most extreme examples of evolution at work. The resultant book, written with bright enthusiasm and describing clearly how the fossil record shows evolution to have occurred, is a wonderful primer about what paleontologists do. Sure, the fossil halls are among the most popular rooms of any natural history museum, but here is why the fossils are important.
The very first chapter has to do with the microbial mats and resultant domed stromatolites, which were the way the Earth manifested life for most of the history that it had life on it. They were the only living things on Earth for much of that time, and they formed the first fossils, from 3.5 billion years ago. There had been candidates for life before such creatures as trilobites existed, but they turned out to be mere scum, or burrows of worms, or other red herrings. The stromatolite fossils, when cut into, reveal concentric ring layers, and while some biologists were sure the layers represented a living process, others pointed out that there was no associated organic matter or preserved cells. There the matter stood until 1956, when in Australia a geologist found the same sorts of domed structures, only they were still alive; colonies of primitive blue-green bacteria were on the surface of the dome, and were responsible for all the rings on the inside. It was an astonishing confirmation that the ancient fossils represented life as far back as fossils are going to show it to us.
The bacteria of the stromatolites formed colonies, but they were not multicellular creatures. Those showed up something like 630 million years ago and because they had no hard parts, their fossils are rare impressions of soft tissue onto sands or muds on the sea bottom. These were the Ediacaran fossils, and it is hard to say what they were. There has been no discovery of a contemporary version, as with the stromatolites, and although we know just a little bit about what they their outsides looked like, we do not know about internal structures. They might be something like sea jellies or worms, but probably they are not like anything we have now. They were the first multicellular creatures, but didn't last long after simple shelled organisms, and then trilobites, showed up.
Organisms began making shells for protection, but it took a long time for them to start doing so. It was originally a mystery about how trilobites suddenly appeared. Special examination was required of layers below trilobites, including bringing the tiny fossil "shellies" out by dissolving the rock around them in acid, or examining microscopically thin sections of rock containing them. These layers had gone unremarked since they didn't look like much in the field, but under the microscope a fantastic array of shelled forms could be seen. They set the stage for the "Cambrian explosion" that produced the trilobites 525 million years ago, but the "explosion" is now known as more of a long fuse, tens of millions of years of a gradual process from soft bodies, to tiny shells, to large hard shelled creatures with articulations.
And so the chapters are laid down, with these earlier ones clearing up at least some questions about the initial origins of life and multicellularity. It takes a while to get to the gee-whiz specimens everybody knows, like T. Rex, and even longer to get to the line that produced humans. Chapters describe how vascular land plants came ashore, and how amphibians did so, and how some mammals went back to sea and became whales. How the turtle got its shell is here, and how the Jurassic Park films have gotten more out of date as they continue to show dinosaurs with skin or scales but without filamentous feathers. (It's OK - there are classic pictures showing the apatosaurus [formerly brontosaurus] as a sluggish, tail-dragging swamp-dweller, even though the interpretation of its fossils now depicts a completely different creature.) Over and over, in one chapter after another, the story is that gaps have been filled. The change from an invertebrate creature to the first unquestioned fish now has a demonstrable fossil lineage, as does the change from fish to amphibians. Each of these chapters, often concentrating on the researchers that made the insights happen, is a little essay on how evolution works and how we learn that it does.
Prothero's book is not a diatribe against those whose religious faith is so strong they just know that the Earth is only about 10,000 years old and that evolution is bunkum. The demonstrations of fossil lineages he makes stand on their own, not as arguments against creationism. Nonetheless, the biblical literalists show up here fairly often. The most amusing such emergence is in a chapter on the largest land animal. Within the chapter Prothero mentions the Mokele Mbembe, an alleged sauropod living in the Congo. Like the Loch Ness Monster, no ecology can account for it, and the only evidence for it is eyewitness reports, which "are the worst possible evidence in a scientific investigation, since human eyes and brains are easily fooled." There are people who go to the Congo looking for this monster, but they are creationist ministers and not wildlife biologists. They have the idea that if they can find a dinosaur, then all the sorts of evidence for evolution as are displayed in this book will be for nothing. It isn't the only example of creationist illogic, but it is among the funnier.
One of the best parts of this entertaining book is that each chapter ends with a "See it for yourself" section, advising readers which museums to go to to see the specimens described, or in some instances, where they can go to collect their own specimens. It is pleasant to reflect that some of the younger readers of this book are going to follow Prothero's exhortations to get into the collection rooms and out to the field, and to further our knowledge of how all the fantastically diverse forms of life came to be.