We Homo sapiens did not tend to be alone. Long ago, there was much more human diversity; Homo sapiens lived with an estimated eight now extinct species human 300,000 years ago. As recently as 15,000 years ago, we shared caves with another human species known as Denisovans. And fossilized remains indicate an even higher number of early human species that once populated the earth before our species followed.
“We have a human species right now, and historically it is very strange,” said Nick Longrich, an evolutionary biologist at the University of Bath in the United Kingdom. “Not so far back, we were not so special, but now we are the only ones left.”
So how many early human species were there?
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When it comes to finding out exactly how many different human species existed, it gets complicated pretty quickly, especially as scientists continue to uncover new fossils that end up being completely separate and previously unknown species.
“The number is increasing, and it will vary depending on who you talk to,” said John Stewart, an evolutionary paleoecologist at Bournemouth University in the United Kingdom. Some scientists claim that the species known as Homo erectus actually consists of several different species, including Homo georgicus and Homo ergaster.
“It’s about the definition of a species and the degree to which you accept variation within a species,” Stewart told WordsSideKick.com. “It can be a bit of an annoying and pedantic discussion, because everyone wants answers. But the truth is, it really depends.”
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 cannot reproduce successfully. Therefore, horses and donkeys, although biologically similar, are not the same species. In recent decades, however, this simplicity has given way to a more complex scientific debate on 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 organisms with similar anatomical features, but that method also has weaknesses. There can be considerable 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 do not have the DNA of all ancient people – the genome of Homo erectus, for example, has never been sequenced, Live Science reported earlier.
It gets even darker when you consider it as much as 2% of the average European DNA comes from Neanderthals and up to 6% of DNA of some Melanesians (indigenous peoples from islands just northeast of Australia in Oceania) come 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 are just a slightly different type of modern humans and interbreeding is the proof, but again the definition of species has gone from just interbreeding.”
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Having taken all this into account, some experts has argued that the concept of a species does not actually exist. But others say that although a cast iron definition of a species is almost impossible to achieve, it is still worth the effort so we can talk about development – including the evolution of our own species – in a meaningful way.
So we move on and know that one species means different things to different people – which of course means that people will disagree about how many human species have ever existed. It is also a question of what constitutes a human being. To answer this question, it helps to understand the word hominin, a large group that includes humans and chimpanzees who go back to their common ancestor.
“The chimpanzee and we have evolved from a common ancestor, “Stewart said. If we decide that humans are all that came after our split from ancient chimpanzees about 6 million to 7 million years ago, then there will probably be a diverse group. The Smithsonian National Museum of Natural History has listed at least 21 human species which is recognized by most researchers. Granted, this is not a complete list; Denisovans, for example, are missing.
Those on the list include Homo sapiens, Neanderthals, the Indonesian hobbit sizes, Homo erectus and Homo naledi. The list also includes other species that existed closer in time to the common ancestor of humans and chimpanzees, and therefore look more like chimpanzees than modern humans. Despite their appearance, these species are still known as early humans. “You can not go back 5 million years and expect them to look like us,” Stewart said.
If the Smithsonian says it’s 21, you can be sure the diversity is much greater, Stewart said. This is because the list fails on the side of caution and picks the species that are close to universally recognized. For example, the newly discovered dwarf human species Homo luzonensis, which is known from only a few bones excavated in an Indonesian cave, is not on the Smithsonian’s list.
Researchers also suspect that there are many other fossilized species that have not yet been excavated. “The list has only grown, and I do not see why that will change,” Stewart said.
Originally published on WordsSideKick.com.