Neanderthals, which disappeared from the archaeological record about 40,000 years ago, have long been considered our closest evolutionary relatives. But since the first discovery of Neanderthal remains in the 1800s, scientists have debated whether Neanderthals constitute their own species or whether they are a subset of our own species, Homo sapiens, which has since gone extinct.
So what does the science say? Specifically, what does the genetic evidence, which did not exist when many early hominins were first discovered, show?
The question of whether Neanderthals can be considered the same species as modern humans is complicated by our understanding of what a species is, Jeff Schwartz, a physical anthropologist and professor emeritus at the University of Pittsburgh, told Live Science.
The most common definition, called the biological species concept, describes a species as a group of individuals that can interbreed in nature and produce viable offspring. But even today, many strange hybrids poke holes in this definition.
\”Horses and donkeys can breed, but the mules they give birth to are infertile, and so the two are considered separate species,\” Schwartz said. But other combinations produce viable offspring. These include ligers (a cross between a lion and a tiger) and beefalo (a cross between a cow and an American bison).
For a long time, scientists didn\’t know whether Neanderthals and modern humans interbred, so this definition wasn\’t particularly enlightening. Instead, early assessments were made based on Neanderthal anatomy, which is so different from H. sapiens that experts could often distinguish the bones of the two groups. For example, Neanderthals had longer, lower skulls, with a bony forehead and less pronounced chin than H. sapiens, and their bodies were thicker.
Because of this, in 1864, Neanderthals were first classified as their own species, H. neanderthalensis. But as even more early human relatives were discovered — such as H. erectus in 1891, H. heidelbergensis in 1907 and H. habilis in 1960 — their relationships to one another became even more complex.
Compared to other species, Neanderthals seemed much more \”human,\” Schwartz said. Recent research has suggested that both groups had similar auditory and vocal abilities, and controversial discoveries indicate that Neanderthals may have buried their dead and made jewelry and art.
In 1962, a group of anthropologists, geneticists and behaviorists met in Austria to draft and vote on an evolutionary history of human relatives based on the species discovered at that time.
Their resulting manuscript, called \”Taxonomy and Human Evolution,\” placed Neanderthals as a subspecies under H. sapiens, or H. sapiens neanderthalensis. \”And that\’s what I was taught when I was in college,\” Schwartz said.
It was only later, in the 1970s and 1980s, that Neanderthals were reclassified as their own species based on newer analyses, and that remains the most common designation seen today.
But a 2010 discovery shook things up again: An international team of dozens of researchers published the first draft of the Neanderthal genome, based on three individuals, and compared it to modern humans.
The authors found hints of a Neanderthal signature in the human genome, suggesting that Neanderthals mixed with modern human ancestors at least 120,000 years ago. Dozens of papers have since confirmed this, finding that mixing occurred over several generations, starting about 47,000 years ago, for about 7,000 years.
\”The implication is clear: Neanderthals and humans were interbreeding,\” Jaume Bertranpetit, an evolutionary biologist at Pompeu Fabra University in Barcelona, Spain, told Live Science. He added that Neanderthals and H. sapiens also interbred with another group of early hominids, the Denisovans. It\’s possible, then, that all three are just different versions of the same species, he said.
Bertranpetit pointed to modern humans as an example. There are notable differences in stature or skin, hair and eye color among people around the world, but from a genetic perspective we are remarkably similar. The amount of genetic variation between any two individuals is just 0.1%, meaning that only one base pair out of every 1,000 will be different. In comparison, a 2010 paper showed that the Neanderthal genome was 99.7% similar to the genomes of five present-day humans.
\”Having big differences in morphology doesn\’t mean you need big differences in genetics as well; it means you need some differences in specific genes,\” Bertrenpetit said. \”So the idea that they were different species doesn\’t make sense to me as a geneticist.\” Schwartz doesn\’t think the genetic evidence has settled the debate, but he has no doubts about the rigor of the work done by other groups.