Saturday, January 20, 2007

Life on a Young Planet: Review

Andrew H. Knoll's Life on a Young Planet: The First Three Billion Years of Evolution on Earth is one of those rare books that can change your, or at any rate, my, picture of reality. I have posted twice before on this book: Eu are Irrelevant and In the Beginning, but the present post is my attempt at a review.

Dinosaurs and mastodons don't wander through these pages, unless you count a cameo or two. Their time was still far in the future when the characters whose story is told here held center stage. The Cambrian Period, which started about 543 million years ago, or a bit less than ten thousand times as long ago as our own species originated, ushered in the Paleozoic Era and the first easily recognizable fossils. It was the first age of animals - not the very first animals, but the first animals with considerable size and complexity. All the animal phyla we now recognize were onstage in the Cambrian, as well as numerous phyla dead and gone. Before the Cambrian, only sponges, jellyfish and other very simple animals left any trace. The three billion years of the fossil record preceding the Cambrian are Knoll's subject, and it is unexpectedly fascinating. A couple of chapters feature the Cambrian, but mainly it looms as an offstage presence, foreshadowed more than depicted.

Knoll is a graceful and vivid writer, and the story he tells is a detective story, as most good science stories should be (and why does that part always get lost in science textbooks, especially those for kids?). The web of evidence from which the past is constructed is an intricate one, built out of chemistry, physics, and biology, classical and molecular, but above all, out of old rock and the traces left on it by life. Because of that intricacy, the book makes some demands on the reader's attention span and memory for pattern, and enough background in biology to know the difference between Eukaryote and Prokaryote would be useful.

The Earth of three and one half billion years ago was different from the one we live in but not quite unrecognizably so. Oceans and continents already existed, and had for 700 million years previously. Those continents have left little of themselves behind though, suggesting, says Knoll, that the great geological engine of plate tectonics might have worked differently in those younger and internally hotter days; able, perhaps, to swallow and digest continents as well as oceanic crust. The earliest, highly suggestive but not quite conclusive, evidence of life dates from then.

Of what does that evidence consist? Microscopic impressions in the rock and organic remains of possible life, and context, context, context. Paleontology is a field science, and paleontologists spend an exciting part of their time clinging to precarious and sometimes icebound cliffs to retrieve their samples, but it is now also a laboratory science. The rocks containing these traces must be sliced in thin transparent sections, scrutinized through powerfull microscopes, subjected to isotopic analysis in a mass spectrograph - life, it seems, is picky about the carbon (and other element) isotopes it uses, and that signature is one of the most definitive. Knoll tells of putting the crucial samples under the microscope, scrutinizing them, and in the end concluding that while a circumstantial case existed, it couldn't be considered conclusive.

A deep skepticism of both one's own ideas and those of others is indispensible in this most vital kind of science. Knoll is expecially good at presenting contrasting ideas and hypotheses - a breath of fresh air after listening to the narrow minded fanaticism of some of the string theory debates. Paleontologists, of course, have a big advantage - they have actual evidence.

A few hundred million years later, the evidence becomes more unambiguous. Casts in the rock preserve more detail, both morphological and biogeochemical. Life, too, becomes more elaborate and presents more features for recognition. The cyanobacteria are one of the main heroes of the story. For two and one half billion years they have thrived almost unchanged - they did much of the heavy lifting of producing the oxygen that was previously lacking in our atmosphere, and they were incorporated in eukaryotic cells to allow plant life to arise.

I won't try to summarize this long elaborate story, but let me mention a couple of themes. Geology set the stage for life, but life became a major player in transforming the atmosphere and the oceans. Occasional cataclysms have shaken and transformed evolution. The photosynthetic production of oxygen was certainly one of these. Others likely had extraterrestial origins - great asteroid impacts, for example. Still others probably originated in the tectonic processes of volcanism and continental drift. Most, like the great ice ages of the late Proterozoic (just before the Cambrian efflorescence) are of uncertain provenance. Those ice ages, incidentally, made the more recent ice ages of the Pleistocene look like midgets. Glaciers advanced even into the tropics, and much, or all, of the ocean surface was frozen to great depths. Cataclysm for one branch of the tree of life may be opportunity for another. Mammals and dinosaurs coexisted for 150 million years, during all of which time the reptiles were clearly dominant. Not until the asteroid wiped out the dinosaurs did the great mammalian expansion happen. Knoll calls this "permissive ecology." When all the good jobs or ecological niches are already taken, it's hard for an evolutionary novelty to get a start.

The penultimate chapter is devoted to paleontology beyond the Earth, and expecially to LAH-84001, the grapefruit sized meteorite from Mars that NASA in 1996 claimed showed evidence of life. His exceptionally clear prose is put to good use explaining the pros and cons, as well as crucial background material (how do we know it came from Mars, for example - it was found in Antarctica.) The years since of intense study of this meteorite have taught science a lot about how to look for signs of life from other planets, but have not been kind to NASA's claim. The claim has not been disproven, but all the lines of evidence upon which NASA made the claim have been cast into doubt.

The final chapter, or epilogue is truly an elegant essay in itself. He touches on many themes, from religion and science to creationism, but concludes with an appeal that we realize the implication of the fact that we, the human race, now play a major role in shaping the future of the planet.

If we can acknowledge our unprecedented role as planetary stewards, we may be able to discharge our responsibility with wisdom and with honor. On this issue, at least, faith and science find common ground. I don't know whether God decreed the passenger pigeon, but if He did, it was not for us to exterminate...

Through our actions or inaction, we decide the world that our grandchildren and great grandchildren will know. Let us have the grace and humility to choose well.

He includes an extensive and lightly annotated list of references and further reading. In case it wasn't obvious, I really liked the book.