We Homo sapiens didn"t used to be alone. Lengthy ago, there to be a lot much more human diversity; Homo sapiens live alongside an estimated eight now-extinct types of human around 300,000 years ago. As newly as 15,000 years ago, we were share caves with an additional human types known as the Denisovans. And fossilized remains suggest an even greater number of early human varieties once occupied Earth prior to our types came along.

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"We have actually one human species right now, and historically, that"s really weird," said Nick Longrich, an evolutionary biologist at the university of bath in the unified Kingdom. "Not that far back, us weren"t the special, but now we"re the only ones left."

So, how countless early human varieties were there?

Related: What"s the an initial species people drove come extinction?

When it comes to figuring out specifically how many distinct types of people existed, it gets facility pretty quickly, especially since researchers save unearthing new fossils that end up being totally separate and also previously unknown species.

"The number is mounting, and also it"ll vary depending upon whom you talk to," claimed John Stewart, an evolution paleoecologist at Bournemouth university in the joined Kingdom. Some researchers argue the the types known together Homo erectus is in fact comprised of several various species, consisting of Homo georgicus and Homo ergaster.

"It"s all around the definition of a types and the level to i m sorry you accept variation within a species," Stewart told Live Science. "It can come to be a slightly irritating and also pedantic discussion, because everyone desires an answer. However the truth is that it really does depend."

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What is a species?

The an interpretation of a types used to it is in nice and also simple: If 2 individuals might produce abundant offspring, they to be from the very same species. For example, a horse and also a donkey deserve to mate to create a mule, however mules can"t successfully reproduce with each other. Therefore, horses and also donkeys, though biologically similar, space not the very same species. In recent decades, however, the simplicity has actually given method to a more complicated scientific debate around how to define a species. Movie critics of the interbreeding an interpretation point the end that no all life reproduces sexually; part plants and also bacteria have the right to reproduce asexually.

Others have argued that we need to define species by grouping together organisms with similar anatomical features, however that method has weaknesses as well. There can be far-ranging morphological variation in between the sexes and also even individuals of the same types in different parts of the world, making the a really subjective method of classifying life.

Some biologists choose to usage DNA to attract the lines in between species, and also with progressing technology, they can do for this reason with increasing precision. But we don"t have actually the DNA the every ancient human — the genome of Homo erectus, for instance, has actually never to be sequenced, Live Science formerly reported.


The skulls of assorted human species (Image credit: Shutterstock)

It gets even murkier as soon as you think about that as lot as 2% the the typical European"s DNA comes from Neanderthals and up to 6% that the DNA of some Melanesians (Indigenous people from islands directly northeast the Australia in Oceania) comes from Denisovans. So, room we a separate species from these ancestors?

"Some world will phone call you that Neanderthals space the same varieties as us," Stewart said. "They’re simply a slightly different form of modern-day humans and the interbreeding is the proof, but again the an interpretation of species has relocated on from just interbreeding."

Related: Why haven"t every primates advanced into humans?

After taking all of this into account, some experts have said that the principle of a species doesn"t actually exist. However others speak that, if a cast-iron definition of a varieties is nearly impossible come achieve, it"s still precious the effort so that we deserve to talk around evolution — consisting of the evolution of ours own species — in a coherent way.

So we muddle on, learning that a species method different things to different human being — i beg your pardon means, of course, that people will disagree on exactly how many species of person have ever existed. It"s likewise a question of what constitutes a human. Come answer this question, it helps to recognize the word hominin, a big group that has humans and chimps going ago to their shared ancestor.

"The chimpanzee and us have progressed from a usual ancestor," Stewart said. If we decide that people are everything that come after our break-up from old chimpanzees around 6 million come 7 million years ago, climate it"s likely to be a varied group. The Smithsonian nationwide Museum of Natural background has listed at the very least 21 person speciesthat are well-known by many scientists. Granted, it"s no a totally finish list; the Denisovans, for instance, are missing.

Those on the list incorporate Homo sapiens, Neanderthals, the Indonesian hobbit-size people, Homo erectus and Homo naledi. The list also includes other types that exist closer over time to the typical ancestor the humans and also chimps, and so look much more like primates than modern-day humans. In spite of their looks, these varieties are still well-known as beforehand humans. "You can"t go back 5 million years and also expect them come look favor us," Stewart said.

—When walk humans find how to use fire?

—How smart were Neanderthals?

—What if Neanderthals had not gone extinct?

If the Smithsonian states there space 21, then you can be sure the diversity is much greater, Stewart said. That’s since the perform errs on the side of caution, choose the types that room close to universally recognized. For instance, the recently discovered dwarf human varieties Homo luzonensis, who is known from just a couple of bones indigenous a cavern in the Philippines, is not consisted of on the Smithsonian"s list.

Researchers also suspect there are many other fossilized species yet to be excavated. "The list has only ever before grown and I don’t watch why that will change," Stewart said.

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Editor"s Note: This article was updated at 9:49 a.m. EST on Jan. 25 to keep in mind that the remains of Homo luzonensis were discovered in the Philippines, not in Indonesia.