"For many years human evolution was looked on as sort of single-minded slog from primitiveness to perfection," says Dr. Ian Tattersall, curator emeritus of anthropology at the American Museum of Natural History. "But as the human fossil record has grown by leaps and bounds, we've found that the pattern seems to have been one of experimentation from the very start. Rather than the culmination of a linear process, humans are much better viewed as the single surviving terminal twig on a very luxuriantly branching tree."
Groups of early hominids—the lineage that split from chimpanzees—first appeared in Africa some 6 million or 7 million years ago. Following this, the hominids diversified considerably, but all eventually went extinct, with the exception of modern humans, or Homo sapiens. "That tells you there's something very special about Homo sapiens," says Tattersall. "It is a very unusual situation for there to be only one hominid species." He theorizes that "it was probably due to a combination of violent confrontation and indirect economic competition with Neanderthals and early humans."
How do we study the human fossil record?
Finding and analyzing fossils is a complex task that typically requires a team of paleoanthropologists (scientists who study the evolution of the human species), geologists, anatomists, and paleoecologists (scientists who study ancient environments)—to name just a few. To piece together the story of how ancient peoples lived and died, these experts analyze artifacts, sediments, plant and animal remains, and DNA evidence.
Noting that the process of studying ancient hominids is the same as for any other kind of mammal, Tattersall observes that luck is as important as logic when it comes to fossil hunting. For example, paleoanthropologists are currently hard at work at Dmanisi, a 1.8-million-year-old site in the Caucasus, the first place outside of Africa where a hominid presence has been documented—and "not a place where anybody would have imagined looking," he notes wryly. Dmanisi was the site of a prosperous town in the Middle Ages that was sacked by Mongols in the 13th century and abandoned. When archaeologists specializing in medieval history came along, they were astonished to find the ancient bones of earlier inhabitants poking out of the walls of the basement pits used to store food.
Tattersall laments "the habit of complaining bitterly that we don't know enough and not appreciating that we have a fairly good fossil record compared to many mammalian families." Fossils are only found in sedimentary rock, and the fact that that humans typically lived near rivers and lakes, which gave rise to sedimentary rock formations, helped protect their remains.
As the human fossil record expands, so does the need for standard descriptions and terminology. Along with his independent inquiries into modern human cognition, Tattersall has been working on a multi-volume documentation of major hominid fossil finds that will serve as a database for further analyses of hominid systematics.
The challenge of early human history
Although exciting hominid finds are being made almost every year, much about human history remains controversial. There are currently several conflicting theories about the origin of the hominid family itself, "and frankly, we don't know what the earliest hominid should look like, so we'd have difficulty recognizing it if and when we find it," Tattersall comments. Paleontologists differentiate species by comparing the same skeletal elements across fossils from different places and periods. Since it's very rare to find complete skeletons, accurate comparisons are rare or nonexistent. Nevertheless, by comparing thousands of hominid bone fragments, paleoanthropologists generally agree that there are 15 or more known hominid species and have grouped them into about five genera, one of which is Homo.
What's a "Human," anyway?
"People have been using the word 'human' since long before they knew they had extinct relatives or, indeed, any close living relatives. So it's really an arbitrary distinction," says the anthropologist. "But I think most scientists would apply the term human to any member of the genus Homo." The earliest fossils that significantly resemble Homo sapiens go back to around 160,000 years ago. Although the new species appears to have behaved much like other hominids of the time, it differed skeletally: It had a tiny face tucked under a very large brain case. (Apes have a big face in front of a very small brain case.) In contrast to its hominid ancestors, some species of which survived for over a million years, over the last 50,000 years this upstart rapidly populated the entire globe.
How did human consciousness evolve?
Along with the dubious distinction of having eliminated all the competition, the trait that most dramatically sets modern humans apart is their symbolic consciousness. Only humans can re-create the world using symbols and manipulate these symbols to create abstract realms of thought. For the past few years, Tattersall has been wrestling with the question of how this nuanced awareness evolved. "Neurologists think about neurons, paleontologists think about fossils, archaeologists think about artifacts, and it's very difficult to put it all together when it comes to studying human consciousness," the anthropologist points out. "The main point to remember is that you've got a fish brain, a reptile brain, a primitive mammal brain, and primate and ape brains inside your head. We don't know exactly how each of these ingredients contributes to the functioning of the human brain, nor to what we subjectively experience as consciousness. But we're unique only because of all those layers supporting consciousness from beneath."
An anatomical change paves the way
Calling the retraction of the face in Homo sapiens "a lucky confluence with a lot of implications," Tattersall points out that "you need a certain morphology of the upper vocal tract and upper airway in order to produce articulate language." Clearly, the emergence of Homo sapiens represented a major biological reorganization, and in the anthropologist's opinion, this set the stage for a major developmental leap: Physiologically capable of speech, the first Homo sapiens were also neurologically capable of symbolic thought. But the new species had to discover its new potential.
Drawing the analogy that birds had feathers for millions of years before using them for flight, Tattersall believes that the capacity for symbolic thought had to be released by a cultural stimulus. "My suggestion, which is not original, is that this stimulus was probably the invention of language—the ultimate symbolic activity," he says. It's a difficult theory to investigate, but anthropologists know that languages are spontaneously invented, and Tattersall's guess is that children could have done so in the context of play. While no one knows when humans started speaking, archaeological evidence of symbolic cognition first appears as early as 75,000 ago in the form of an engraved piece of South African ochre, and it is indisputably present by 35,000 years later, in the European Cro Magnons' musical notations and cave paintings.
Are humans still evolving?
Tattersall doesn't think so: "As far as we know, new traits only get fixed in populations that are small and genetically unstable." Homo sapiens and its predecessors existed in small, discrete populations that were constantly being isolated by fluctuating environments, ideal conditions for new mutations and modifications to be incorporated into the gene pool. But Earth's 6 billion inhabitants are unprecedentedly mobile, and anyone can reproduce with anyone else on the planet. "Under those conditions you can't expect any meaningful biological novelties to become fixed," he says.
"We can't wait for evolution to improve us and save us from our follies; we'll have to learn to live with ourselves the way we are," Tattersall points out. But that's not the end of the story: "In a real sense we have replaced biological with cultural innovation, and all the exciting developments in the future will involve finding new ways to exploit this remarkable capacity we already possess."
This essay was developed for the AMNH online course Evolution, part of Seminars on Science, a program of online professional development courses for educators.
Human evolution: The evidence (American Museum of Natural History)