From the Beginning
Flannery, Maura C., The American Biology Teacher
There's a tremendous amount of evidence for evolution, but biologists are always looking for more. As with any delving into the past, this isn't easy to do. Time erases evidence. A number of wonderful sites of prehistoric cave art have been found over the years, from Altamira in the 19th century to Chauvet in the 1990s. But the experts still haven't come up with a plausible explanation for why this art was created (Curtis, 2006). Questions still remain: were these images meant to celebrate the diversity of life or to bring blessing upon a future hunt? Such questions are what make history both a frustrating and a fascinating endeavor, and no part of history is more frustrating or fascinating than investigating early life on earth. If it's hard to piece together what was going on in caves 20 or 30 thousand years ago, it's not surprising that figuring out what occurred 3 or 4 billion years ago would be much more difficult. The amazing thing is that it isn't totally impossible. Biologists, chemists, physicists, and geologists have worked together to come up with some plausible scenarios for the early years of life on earth. Sure, there's still much controversy about some of their explanations, but there has also been a lot of progress since the experiments by Stanley Miller and Harold Urey in the 1950s in which they attempted to recreate the chemical environment of the early earth. In this column, I want to explore several lines of evidence that together give us at least a sketchy view of what early life was like. That's not bad, considering that many of us can't trace our ancestors back more than two or three generations.
* Early Earth
Physicists and geologists agree that the earth is about 4.5-4.6 billion years old. There is also good evidence that in its early years, the earth was a hot and turbulent place. When it was only about 100 million years old, it suffered what Lynn Rothschild (2009: p. 335) calls its "worst day ever" when it was hit by a "rogue" planet. This impact resulted in the formation of the moon and also melted the earth's surface. After that, there were repeated impacts, especially during the late heavy-bombardment era, from 4.1 to 3.9 billion years ago, which left the earth hot and the oceans steamy. It also left little time for the evolution of early life, if, as some argue, that life was significant enough to leave evidence in rocks that are 3.8 billion years old. A hundred million years may seem a long time to evanescent creatures like ourselves, but Darwin was upset when the physicist Lord Kelvin calculated that the earth was only about a hundred million years old. Few, if any, rocks remain from the bombardment period, yet there are a few about 3.8 billion years old that some geologists think contain the remnants of early life. So two questions arise: how solid is this evidence, and how could life arise so rapidly when our experiments to replicate the conditions of the early earth suggest that synthesis of macromolecules would not be an easy or swift process?
Welcome to the world of early evolution! It definitely seems long on questions and short on answers, but things are hardly hopeless, and new evidence keeps being unearthed. Geologists have identified rocks in the Hudson Bay area of Canada that they date to 4.3 billion years ago. This estimate is on the basis of a new dating method using the radioactive decay of samarium-146 (Kerr, 2008). If the calculation is verified, this would make the rock part of the earth's protocrust, which formed from the earliest mantle rock. This is a first, and it's too early to tell if these results will hold up, but there are very old zircon inclusions in "younger" rocks, those dating from about 3 billion years ago (Chang, 2008). Some of these zircons may have formed almost 4.4 billion years ago. The oxygen isotopes in the zircons suggest that water was present as they formed, which indicates that the earth may not have been as hot, at least in some areas, as earlier thought. …