Surprisingly, there is no official answer. Science has yet to agree on a formal description for our genus, Homo, or our species, sapiens.
It’s not for lack of trying. There are actually several suggested definitions for the human genus – and an astonishingly broad range of opinions over what does and does not belong within it.
Talk to some scientists and you’ll be told that the genus Homois little more than 100,000 years old and excludes even the most famous prehistoric “humans”, the Neanderthals. But others say our human genus actually has a history stretching back about 11 million years old and includes not only living people and extinct Neanderthals, but also chimpanzees and even gorillas.
How can there be so much disagreement on such a fundamental issue? And, more importantly, which definition of the human genus is the right one?
“That’s the $64,000 question,” says Jeffrey Schwartz at the University of Pittsburgh in Pennsylvania, US.
The problem arguably begins with the 18th-century biologist Carl Linnaeus, who was the first to standardise the way species and genera are named and defined. He named thousands of species in his seminal 1735 book Systema Naturae, but when it came to our genus, he got a bit metaphysical.
The basic wisdom is that brain size got bigger
When he named each animal genus, Linnaeus carefully noted its defining physical features. But under Homo he simply wrote “nosce te ipsum”: a Latin phrase meaning “know thyself”.
Perhaps Linnaeus thought humans were so obviously different from other animals that a formal physical definition was unnecessary. Or perhaps he was referring to the fact that humans are the only animals with the self-awareness to appreciate their own existence.
Either way, his choice of words implied that humans are fundamentally different from everything else.
It is an understandable mistake: he was working over a century before the publication of Charles Darwin’s theory of evolution by natural selection, which made it clear that humans are a part of the animal kingdom. But researchers like Schwartz argue that Linnaeus’s decision may help explain why the human genus continues to be so difficult to define.