ʜᴏᴡ ᴍᴀɴʏ ᴇᴀʀʟʏ ʜᴜᴍᴀɴ sᴘᴇᴄɪᴇs ᴇxɪsᴛᴇᴅ ᴏɴ ᴇᴀʀᴛʜ?

It depends on your definition of human.

We Homo sapiens didn\’t used to be alone. Long ago, there was a lot more human diversity; Homo sapiens lived alongside an estimated eight now-extinct species of human about 300,000 years ago. As recently as 15,000 years ago, we were sharing caves with another human species known as the Denisovans. And fossilized remains indicate an even higher number of early human species once populated Earth before our species came along.

“We have one human species right now, and historically, that\’s really weird,” said Nick Longrich, an evolutionary biologist at the University of Bath in the United Kingdom. “Not that far back, we weren\’t that special, but now we\’re the only ones left.”

So, how many early human species were there?

When it comes to figuring out exactly how many distinct species of humans existed, it gets complicated pretty quickly, especially because researchers keep unearthing new fossils that end up being totally separate and previously unknown species.

“The number is mounting, and it\’ll vary depending on whom you talk to,” said John Stewart, an evolutionary paleoecologist at Bournemouth University in the United Kingdom. Some researchers argue that the species known as Homo erectus is in fact made up of several different species, including Homo georgicus and Homo ergaster.

“It\’s all about the definition of a species and the degree to which you accept variation within a species,” Stewart told Live Science. “It can become a slightly irritating and pedantic discussion, because everyone wants an answer. But the truth is that it really does depend.”

What is a species?

The definition of a species used to be nice and simple: If two individuals could produce fertile offspring, they were from the same species. For example, a horse and a donkey can mate to produce a mule, but mules can\’t successfully reproduce with each other. Therefore, horses and donkeys, though biologically similar, are not the same species. In recent decades, however, that simplicity has given way to a more complex scientific debate about how to define a species. Critics of the interbreeding definition point out that not all life reproduces sexually; some plants and bacteria can reproduce asexually.

others have argued that we should define species by grouping together organisms with similar anatomical features, but that method has weaknesses as well. There can be significant morphological variation between the sexes and even individuals of the same species in different parts of the world, making it a very subjective way of classifying life.

Some biologists prefer to use DNA to draw the lines between species, and with advancing technology, they can do so with increasing precision. But we don\’t have the DNA of every ancient human — the genome of Homo erectus, for instance, has never been sequenced

It gets even murkier when you consider that as much as 2% of the average European\’s DNA comes from Neanderthals and up to 6% of the DNA of some Melanesians (Indigenous people from islands directly northeast of Australia in Oceania) comes from Denisovans. So, are we a separate species from these ancestors?

“Some people will tell you that Neanderthals are the same species as us,” Stewart said. “They’re just a slightly different type of modern humans and the interbreeding is the proof, but again the definition of species has moved on from just interbreeding.”

Related: Why haven\’t all primates evolved into humans?

After taking all of this into account, some experts

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