September 2007

“Fossils challenge old evolution theory,” proclaimed Fox News, while the Salt Lake Tribune, bragged that “University scientists defy evolution view!” From the headlines trumpeted in some media outlets, one might imagine evolution as a theory in crisis — publishers struggling to rewrite textbook chapters before print deadlines, biologists running from their labs, tearing their hair. In fact, nothing could be further from the truth. Last month, when scientists published a description of newly discovered hominid fossils and suggested that they might prompt a minor revision of the human family tree, biologists and paleoanthropologists considered the additional evidence with interest and the authors’ interpretation with a healthy skepticism typical of science. Since events that occurred more than a million years ago are difficult to reconstruct, paleontologists welcome additional lines of evidence relevant to those details and expect our understandings of them to be refined as evidence accumulates. The fossils suggested reexamining a few hypotheses about hominid evolutionary history, but nothing about them called evolutionary theory into question. In fact, the new evidence fit well with everything we know about how evolution works. Unfortunately, some influential media outlets’ quick gloss on this discovery exaggerated its implications and distorted the true nature of science…

Where's the evolution?

The science behind last month’s sometimes sensational headlines is simple enough. In 2000, an international team of researchers led by Dr. Meave Leakey discovered two fossils in Kenya — both from now-extinct members of our own genus Homo. The 1.44 million year old jawbone was hypothesized to have belonged to Homo habilis, a big-brained, tool-toting vegetarian — and the earliest member of Homo to have evolved. The slightly older 1.55 million year old skullcap was attributed to Homo erectus, a likely candidate for the direct ancestor of modern humans. After seven years of study, the Leakey team reported their findings last month. They interpret the fossil evidence as supporting two separate hypotheses:

• First, because the new H. erectus fossil is unusually small compared to other adult H. erectus fossils, the team argues that in this species, males must have been substantially larger than females — and that this particular fossil must have come from a female. Many species alive today (e.g., gorillas and elephant seals) have this sort of gender-based size difference, so it would not be surprising if one of our own ancestors did too.

• Second, the new H. habilis fossil is unusually recent — about 200,000 years more recent than the next youngest H. habilis fossil known. If the team is correct in their identification and dating of the fossil, it would mean that H. habilis and H. erectus must have lived alongside one another for almost a half million years — and that could shake up biologists’ views of how these species were related. Some evidence suggests that H. habilis slowly evolved into H. erectus and that H. erectus, in turn, evolved into modern humans (hypothesis A below). However, that scenario doesn’t mesh neatly with the new evidence suggesting that habilis and erectus lived at the same time. Partly on the basis of their discovery, the Leakey team argues that i>habilis and erectus must have instead been evolutionary cousins, both descendents of a third species (hypothesis B below).

Since science relies on testing ideas with multiple lines of evidence, all of these hypotheses are still being investigated. But even if more evidence accumulates and these new ideas become widely accepted by the scientific community, the change would NOT — contrary to last month’s headlines — constitute a “shakeup” or challenge to evolutionary theory. Accepted scientific theories, like evolution, are well tested, thoroughly supported explanations for a broad range of natural phenomena. They encompass many smaller ideas and hypotheses, and changes to these details reflect a refinement (not an overthrow) of the over-arching theory. If our own family tree does indeed need to be revised based on these new fossils, or if H. erectus did turn out to be more variable in size than we’d previously thought, it would represent a small change in a specific portion of our knowledge about the history of life on Earth — but it would change none of the central ideas of evolutionary theory: that life on Earth has evolved, that different species share common ancestors, and that natural selection and other processes lead to evolutionary change.

Interestingly, even the scientists involved with this research anticipated its misinterpretation. An article from the Associated Press, ironically (and misleadingly) titled “Fossils Challenge Old Evolution Theory” by Fox News, explains the concern:

Susan Anton, a New York University anthropologist and co-author of the Leakey work, said she expects anti-evolution proponents to seize on the new research, but said it would be a mistake to try to use the new work to show flaws in evolution theory.“This is not questioning the idea at all of evolution; it is refining some of the specific points,” Anton said. “This is a great example of what science does and religion doesn’t do. It’s a continous [sic] self-testing process.”

Biologists expect our view of life’s history to change over time — after all, that’s what science does: builds new knowledge of the natural world, continually improving and refining our previous understandings. Scientific theories can be challenged and overturned, but it takes a preponderance of evidence, a lot of careful scientific work, and an alternative theory that is a more compelling and useful explanation of how the world actually works. Evolution, however, is far from a theory in crisis. Its central ideas are supported by the weight of scientific evidence available (including the fossils discovered by the Leakey team), have been investigated by scientists the world over for more than a century, and continue to provide useful insights into fields as diverse as conservation biology, agriculture, computer programming, and human health.