Ancient DNA and Neanderthals
While DNA can be used to understand aspects of biology and evolution, the fact that DNA is a fragile molecule and decays over time has made it difficult to use DNA to learn more about extinct species. After decades of work, scientists can now use ancient DNA to understand aspects of the biology of our closest extinct relatives, the Neanderthals (Homo neanderthalensis) such as their appearance, physiology, speech capability, and population structure, as well as their phylogenetic relationship with modern humans, our own species (Homo sapiens).
Neanderthals were the first fossil hominin species discovered by scientists in 1856. The Neanderthal lineage has been the source of much debate within the anthropological community, but the consensus now is that the most likely common ancestor of Neanderthals and modern humans (at least with the current fossil record) is Homo heidelbergensis. The Neanderthal and modern human lineages probably began to diverge about 500,000 years ago, with the ancestral population to Neanderthals traveling to Europe and the Middle East and the ancestral population to modern humans remaining in Africa for roughly another 400,000 years. Neanderthals had plenty of time to develop cold-weather adaptations to allow their bodies to retain as much heat as possible in the frigid European climates. Their bodies were stockier and their limbs slightly shorter and more robust than their modern human counterparts. Despite this difference, Neanderthals and modern humans looked very similar and occupied similar ecological niches when their habitats overlapped. One question remains central to the study of Neanderthals and modern humans: if we were so similar, why did the Neanderthals become extinct and while modern humans thrived?
We can use the DNA from fossil Neanderthals to approach this, and many other questions, such as: What was the relationship between Neanderthals and anatomically modern Homo sapiens? Did Neanderthals and anatomically modern humans interbreed? Did Neanderthals contribute to the modern genome? How much? What do the Neanderthal genes that have been identified in the modern human genome actually do? Scientists answer these questions by comparing samples of Neanderthal nuclear and mitochondrial DNA to those of modern humans, even comparing them gene to gene.
Denisovans: Another Human Relative
Scientists have also found DNA from another extinct hominin population: the Denisovans. The only remains of the species that have been found to date are a single fragment of a phalanx (finger bone) and two teeth, all of which date back to about 40,000 years ago (Reich 2010). This species is the first fossil hominin identified as a new species based on its DNA alone. Denisovans are relatives of both modern humans and Neanderthals, and likely diverged from these lineages around 300,000 to 400,000 years ago. You might be wondering: If we have the DNA of Denisovans, why can’t we compare them to modern humans like we do Neanderthals? Why isn’t this article about them too? The answer is simply that we don’t have enough DNA to make a comparison. The three specimen pool of Denisovans found to date is statistically far too small a data set to derive any meaningful comparisons. Until we find more Denisovan material, we cannot begin to understand their full genome in the way that we can study Neanderthals.
Neanderthals and modern humans shared habitats in Europe and Asia
We can study Neanderthal and modern human DNA to see if they interbred with modern humans
We can study the DNA of Neanderthals because we have a large enough Neanderthal sample size (number of individual Neanderthals) to compare to humans