The science journal Nature reports
that new research shows that archaic humans such as Neanderthals may be extinct as a separate species but that they may still live on in the DNA of modern humans.
Research presented on 17 April 2010 at the annual meeting of the American Association of Physical Anthropologists in Albuquerque, New Mexico shows that a genetic analysis of 1,983 people from 99 populations in Africa, Europe, Asia, Oceania and the Americas indicates that extinct species such as the Neanderthals have interbred twice with the ancestors of modern humans.
Sarah Joyce, a doctoral student who works with Jeffrey Long, analysed 614 microsatellite
positions. She created an evolutionary tree to help explain the genetic variation in the microsatellites. It turned out that the best way to explain this variation was the occurrence of two periods of interbreeding between humans and an archaic species, such as Homo neanderthalensis
or Homo heidelbergensis
Using estimated rates of genetic mutation and data from the fossil record, the researchers think that the periods of interbreeding may have occurred around 60,000 years ago in the eastern Mediterranean and around 45,000 years ago in eastern Asia, both after the first migration of Homo sapiens
out of Africa. That would explain why Long and his team did not find evidence of interbreeding in the modern Africans that were included in the study.
The researchers think that the population that resulted from the first interbreeding then migrated to Europe, Asia and North America, and that a second interbreeding with an archaic population in eastern Asia then altered the genetic makeup of people in Oceania.
The hypothesis may not have to wait long to be tested. Svante Pääbo and his colleagues from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, are preparing to publish the Neanderthal genome in the near future. Pääbo's studies has more or less ruled out interbreeding so far, but because those were not based on a complete genome, they are less definitive.
Furthermore, the fossil record contains fossils that are compatible with such interbreeding. The Pääbo team and Russian colleagues have recently published the mitochondrial genome of an archaic human who lived in the Altai Mountains, near ancient Asian trade routes in southern Siberia.
This DNA came from a piece of finger bone for which the species has not been defined. It could be Homo neanderhalensis
or a hitherto unknown Homo
species, or another archaic species such as Homo erectus
that migrated to Oceania by 1.8 million years ago.
The Pääbo team has said that the bone came from an individual that lived in Denisova Cave, some 30,000 - 48,000 years ago, near a place where both modern humans and Neanderthals lived. Other researchers have disputed the age of the bone, however. They think that the sediments of the cave may have undergone transformations, indicating that the layer in which the bone was found may be older.
This genetic model that shows species interbreeding also leads to new questions about the range of species like Homo heidelbergensis
. Human skeletons have been found at Lake Mungo in New South Wales, Australia, that presented robust features that may be the result of interbreeding. They are thought to be more than 20,000 years old.
The reason I thought this story was interesting is because it is a good example of how science works. Science is not a body of immutable knowledge. Everything is tentative, everything can be questioned at any time.
The more solid evidence there is for a hypothesis (the migration from Africa, for example), the more solid evidence it takes to change or even reverse that hypothesis. If the evidence is less solid (the age of the bone in Denisova Cave), it is easier to question its validity.
At the same time, the scientific method is flexible enough to allow completely new ideas to be presented. The evolutionary tree, created on the basis of the data, suggest something important happened. It is then speculated that interbreeding could explain the anomalies in the data. This is accepted as a working hypothesis, to be confirmed or disproven by data that must still come in.
Science is about dealing with uncertainty and reasoning on the basis of uncertainties and being prepared to adjust, adapt or even throw out a hypothesis that cannot be maintained in light of new evidence.
Compare this flexible, evidence-based open-mindedness with the paralogical wishful thinking that is so popular outside scientific circles: "I am a smoker since 40 years. I am not dead. I don't have lung cancer. Therefore, smoking is perfectly safe, and nothing you anti-smokers can tell me will change that. You are just too close-minded following your science religion"