How many early human species existed on Earth?

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How many early human species existed on Earth?
By Benjamin Plackett - Live Science Contributor 5 hours ago

It depends on your definition of human.


Australopithecus skull
An Australopithecus skull
(Image: © Jose A. Bernat Bacete via Getty Images)
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?

Related: What's the first species humans drove to extinction?

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, Live Science previously 
reported.

skulls of early human species

The skulls of various human species  (Image credit: Shutterstock)
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 have argued that the 
concept of a species doesn't actually exist. But others say that, while 
a cast-iron definition of a species is almost impossible to achieve, 
it's still worth the effort so that we can talk about evolution — 
including the evolution of our own species — in a meaningful way.

So we muddle on, knowing that a species means different things to 
different people — which means, of course, that people will disagree on 
how many species of human have ever existed. It's also a question of 
what constitutes a human. To answer this question, it helps to 
understand the word hominin, a large group that includes humans and 
chimps going back to their shared ancestor.


"The chimpanzee and us have evolved from a common ancestor," Stewart 
said. If we decide that humans are everything that arrived after our 
split from ancient chimpanzees about 6 million to 7 million years ago, 
then it's likely to be a diverse group. The Smithsonian National Museum 
of Natural History has listed at least 21 human species that are 
recognized by most scientists. Granted, it's not a totally complete 
list; the Denisovans, for instance, are missing.

Those on the list include Homo sapiens, Neanderthals, the Indonesian 
hobbit-size people, Homo erectus and Homo naledi. The list also includes 
other species that existed closer in time to the common ancestor of 
humans and chimps, and so look more like chimpanzees than modern-day 
humans. Despite their looks, these species are still known as early 
humans. "You can't go back 5 million years and expect them to look like 
us," Stewart said.

RELATED MYSTERIES
—When did humans discover how to use fire?

—How smart were Neanderthals?

—What if Neanderthals had not gone extinct?

If the Smithsonian says there are 21, then you can be sure the diversity 
is much greater, Stewart said. That’s because the list errs on the side 
of caution, picking the species that are close to universally 
recognized. For instance, the recently discovered dwarf human species 
Homo luzonensis, who is known from just a few bones unearthed in an 
Indonesian cave, is not included on the Smithsonian's list.

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

Originally published on Live Science.

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