This technological capability and skeletal fossil analysis kicked into full gear a debate that pits one set of evidence against another in a quest to understand the evolution of our species. The controversy compares the idea that our ancestors all came out of a common community in Africa and replaced other hominids with the competing theory of multiregional evolution as species interbred.
“From my perspective, and that of the geneticists I know, the majority of data support a recent Africa origin model,” said Chris Stringer, a British anthropologist and a leading proponent of the model. Recent is relative - 200,000 years as compared to 2 million years in the multiregional, or "candelabra," model.
The theory of an African cradle holds that anatomically modern humans first developed in East Africa 200,000 years ago and then spread across Africa, Europe and Asia, said Erin Waxenbaum, visiting assistant professor of anthropology at Northwestern University in Evanston. In the process, our species eventually drove other hominids to extinction by either out-competing them for resources or becoming their predators.
Since DNA has the capability to mutate, creating changes that organisms can pass to their descendents over time, the first anatomically modern humans were probably different than the species we are today. Studying modern differences in DNA nucleotides, or DNA sequences, between individuals in Africa illustrates this potential replacement scenario, Waxenbaum said.
Human migration is usually characterized by a small migrant group leaving a larger settled community, Waxenbaum added. Those that leave would inevitably carry with them only a small portion of the genetic diversity characterizing the entire group. The departing group’s DNA will experience mutation and adaption to their new environment, but their genetic diversity will still be smaller than the diversity of the entire home group.
“That’s what we see today,” Waxenbaum said. Looking not only at nuclear DNA but mitochondrial DNA, studies have shown that of all human groups anywhere in the world, the greatest, widest variety of human genetic variation is within the Sub-Saharan Africa region. Nuclear DNA is found in the nucleus of each cell while mitochondrial DNA is found in the cellular mitochondria that converts food to energy.
Research demonstrating this pattern of genetic variation between modern human groups was published earlier this year by University of Pennsylvania researcher Sarah Tishkoff and colleagues in the journal Science Express.
The second theory, known as the Multiregional Continuity model, is based on fossil analysis as well. It concludes that, although humans stem from common hominid parents in Africa, humans simultaneously evolved in different regions of the world. The current debate stems from a misinterpretation of the multiregional model, said Milford Wolpoff, professor of anthropology at the University of Michigan and a leading proponent of the multiregional theory.
According to Waxenbaum, fossil evidence shows that pre-modern homo sapiens appeared relatively quickly throughout Africa and into other parts of the world.
“We assume it would take a long time to march your way from Africa to Asia, so why did these fossil records develop so quickly unless homo sapiens evolved in these regions independently,” she said
The problem with this view, Waxenbaum said, is how the same modern species could develop in such different environments, given the great continental divides of Africa, Europe and Asia. If they evolved totally independently of one another, it's unlikely they all could develop into one species that can interbreed, she said.
But the multiregional theorists say fossil records show that ancient hominid species moved around a lot and interbred throughout the world to produce modern humanity in varied places, said Alan R. Templeton, professor of Biology, Genetics, and Biomedical Engineering at Washington University in St. Louis.
The process began with the spread of the hominid homo erectus from Africa about 2 million years ago and ended with today’s homo sapiens. Researchers such as Wolpoff cite the features of some Neanderthal skeletons found in Europe as showing evidence of interbreeding between Neanderthals and ancient homo sapiens.
Some scientists question evidence offered for each theory. Templeton said his own research statistically shows a low to negative probability that older Eurasian populations were completely replaced by homo sapiens migrating from Africa
But DNA analysis is adding a new dimension to the debate. Some geneticists would disagree strongly with Templeton’s conclusions, said Stanley Ambrose, anthropology professor at the University of Illinois at Urbana-Champaign.
Ambrose disagrees that Neanderthal skeletons show evidence to support regional interbreeding.
Those that argue for hybridization cannot point to a single gene that is clearly derived from Neanderthals, Ambrose said. In fact, research at the Max Planck institute showed that Neanderthals contributed very little variation to modern human populations. Neanderthals and humans did coexist for about 10,000 years, Ambrose said, but DNA evidence proving that the two species reproduced has not been found.
“If they did hybridize, this has not resulted in any population that survived to the present,” Ambrose said.
In addition, if there was no replacement and instead interbreeding to eventually produce our modern species, then scientists would expect to find a lot more genetic diversity today, said Riphan Malhi, assistant professor of Anthropology at the University of Illinois at Urbana-Champaign.
But while some supporters of the multiregional model claim that ethnic and racial diversity in the world today reinforces their case, researcher Richard Lewontin showed in the 1970s that there is more genetic diversity within one racial group than between races.
“In reality, races are very strong social and cultural constructs not necessarily based in biology” even though people look different, Waxenbaum said.
Research continues. Fossils are still being found today, Waxenbaum said, some of the earliest hominid ancestors have been found as recently as the last few years. In addition, the Max Planck Institute used its extracted DNA to generate a "first draft" sequence of the Neanderthal genome this year.
Ultimately, while both research and theories abound, Waxenbaum said, scientists still do not have all the pieces to fill the gaps.
“It’s an ongoing process,” she said.
According to Stringer, it is better not to think in absolutes. Aspects of both models may prove to interplay.
“If one Neanderthal interbred with a modern human somewhere, would that refute the replacement theory?” said Stringer.