Fossils

Ancient microbial fossils are exceptionally rare. Those fragile fragmentary clues may help us bridge the vast gulf between life and non-life. Ancient rocks may reveal key steps in the origin of life. It is true that they are difficult to find, but it’s well worth the effort, because Earth’s earliest fossils provide us with unique information about life’s beginnings. For one thing they provide evidence regarding the size and shape of ancient life. In addition, they reveal a lot about the timing of life’s origin.

In thinking about life’s origins is critical to know roughly when it happened, and how quickly. After all, if life arose relatively quickly, then the process may be relatively easy. A fast emergence of life supports the idea that life is common in the universe. Our best guesses about the timing of life’s origin is in the form of a bracket in time. Fossil evidence suggests that 3.5 billion years ago life had established a firm foothold on Earth. Some geologists claim that Earth was a living planet perhaps 3.85 billion years ago. We don’t know exactly when life arose. In any case, life’s emergence was rapid, at least on a geological time scale.

Paleontologists devote their lives to looking for fragmentary signs of life in rocks. Paleontologists, perhaps more than scientists in any other discipline, can generate attention and appear on headlines. The media loves to relate stories about discoveries in ancient rocks. We’ve read stories about the discovery of history’s biggest sharp, or the most massive dinosaur, or the oldest human skull. These stories inspire the public imagination.

Famous paleontologist William Schopf announced in 1993 the discovery of Earth’s oldest fossils. Schopf claimed to have identified actual single cells of several different species. These cells may have been preserved for 3.4 billion years in rocks from Western Australia. What’s even more surprising is that these cells occurred in filament-like chains strongly reminiscent of those formed by modern microbes that are photosynthetic. These modern photosynthetic cells have the advanced chemical capability to harvest sunlight. Schopf hinted that these species might have also been chemically advanced.

For one thing, this 1993 discovery wasn’t really all that new. Ancient microbes in rocks from the same region of Australia and of similar age have been known since the late 1970’s. What’s more, Schopf had published a report on these several years earlier. It’s not at all clear why the 1993 paper attracted so much publicity compared to the earlier papers.

I should also state that many scientists think that the study of these ancient rocks tell us absolutely nothing about the origins of life. According to these scientists, these remains represent organisms that were already so advanced, that we can’t deduce anything about the transition from non-life to life.

Schopf’s claim was extraordinary, and scientists consequently demanded extraordinary evidence. In this case, however, the geological community was ready to accept Schopf’s claim. He had spent decades establishing a reputation as one of the world’s leading experts in finding and describing ancient fossils.

What Schopf’s new finding in Australia surely accomplished was to push back the record for the world’s oldest life by a few hundred million years. These ancient rocks reveal a host of tiny spheres, discs, rods and chains that appear to be just like modern bacteria. The discovery of unambiguous microfossils in several ancient rocks led to the first prominent publication on these supposed microbes, and they are now found on most biology textbooks.