We Homo sapiens I didn’t used to be alone. Long ago, there was much more human diversity; Homo sapiens lived next to an estimate eight species now extinct of humans about 300,000 years ago. 15,000 years ago, we were sharing caves with another human species known as Denisovans. And the fossilized remains indicate an even greater number of primitive human species that populated the Earth before the emergence of our species.
“We have a human species now and, historically, this is very strange,” said Nick Longrich, an evolutionary biologist at the University of Bath in the United Kingdom. “Not too long ago, we weren’t that special, but now we are the only ones left.”
So, how many primitive human species were there?
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When it comes to finding out exactly how many different species of humans existed, it gets complicated very quickly, especially as researchers continue to dig up new fossils that end up being totally separate and previously unknown species.
“The number is increasing and will vary depending on who you talk to,” said John Stewart, an evolutionary paleoecologist at the University of Bournemouth in the United Kingdom. Some researchers argue that the species known as Homo erectus is actually composed of several different species, including Homo georgicus and Homo ergaster.
“It’s all about defining a species and the degree to which you accept variation within a species,” Stewart told Live Science. “It can become a somewhat irritating and pedantic discussion, because everyone wants an answer. But the truth is, it really depends.”
What is a species?
The definition of species used to be pleasant and simple: if two individuals could produce fertile descendants, they would be of the same species. For example, a horse it is a donkey they can mate to produce a mule, but mules cannot reproduce with each other. 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 about how to define a species. Critics of the definition of crossing point out that not all life reproduces sexually; some plants and bacteria can reproduce asexually.
Others argued that we should define species by grouping organisms with similar anatomical characteristics, but this method also has weaknesses. There may 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 the advancement of technology, they can do this with increasing precision. But we don’t have the DNA of every ancient human being – the genome of Homo erectus, for example, has never been sequenced, Live Science previously reported.
It gets even darker when you consider that as much as 2% of the average European’s DNA comes from Neanderthals and even 6% of the DNA from some Melanesians (indigenous people from the islands directly to the northeast of Australia in Oceania) comes from Denisovans. So, are we a separate species from these ancestors?
“Some people will say that Neanderthals are the same species as us,” said Stewart. “They are just a slightly different type of modern humans and breeding is proof, but again the definition of species has changed from just breeding.”
Related: Why didn’t all primates evolve into humans?
After taking all this into account, some experts argued that the concept of species doesn’t really exist. But others say that while a cast iron definition of a species is almost impossible to achieve, it’s still worth the effort for us to talk about evolution – including the evolution of our own species – in a meaningful way.
So we remain confused, knowing that one species means different things to different people – which means, of course, that people will disagree about how many species of humans have ever existed. It is also a question of what constitutes a human being. To answer this question, it is useful to understand the word hominine, a large group that includes humans and chimpanzees that date back to their common ancestor.
“O chimpanzee and we evolved from a common ancestor, “said Stewart. If we decide that humans are everything that came after our separation from the ancient chimpanzees about 6 million to 7 million years ago, then it is likely to be a diverse group. Museum Smithsonian National Museum of Natural History has listed at least 21 human species that are recognized by most scientists. Granted, it is not a complete list; Denisovans, for example, are missing.
Those on the list include Homo sapiens, Neanderthals, the Indonesian hobbit-sized people, Homo erectus and Homo naledi. The list also includes other species that existed closer 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 the first humans. “You can’t go back 5 million years and expect them to look like us,” said Stewart.
If the Smithsonian says there are 21, you can be sure that the diversity is much greater, said Stewart. This is because the list errs on the side of caution, choosing the species that are almost universally recognized. For example, the recently discovered dwarf human species Homo luzonensis, which is known for only a few bones dug up in a cave in Indonesia, is not included in the Smithsonian’s list.
The researchers also suspect that there are still many other fossilized species to be excavated. “The list just grew and I don’t see why that is going to change,” said Stewart.
Originally published on Live Science.