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To Be Species or Not

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Thinking, probably, about the history of humanity – Image credit: https://commons.wikimedia.org/wiki/File:In_Thought_…_(3020466221).jpg;

We want to be sure that the history of mankind is a history of us, Sapiens. First, that means we need to understand how to differentiate our species, Homo sapiens, from other human non-Sapiens species. Secondly, we also have to be sure that during the ongoing evolution we, Homo sapiens, were not evolved yet into several different species. In this blog post, we will look into how scientists differentiate one species from another.

Too Old to Fail

We know that in the 18th century Carl Linnaeus introduced a new system of classification in biology. The basic unit of this system is a species. Species are grouped together into a genus, genera are grouped into families, and so on.

     The process, by which one species splits into two distinct species, is called speciation. There are different ways how it could happen.

       There are over 8 million different living species. That is a huge number. About one-quarter of those species were classified using Carl Linnaeus’s classification system. That species’ classification system is in use for over two centuries. It will stand no matter how many flaws it has and no matter how it could be modified in every specific case. 

New Wine in Old Wineskins

Therefore it makes sense to provide clarification on how some particular species are identified. For a long time, species were defined by scholars as the largest group of organisms, in which any two individuals of the appropriate sexes can produce fertile offspring. However, that definition has a lot of shortcomings. Some species do not have sex at all; fossil reproduction cannot be examined, etc.

Liverwort plant, which reproduces asexually. Image credit: https://commons.wikimedia.org/wiki/File:Caduco.jpg

      Yet, that definition is broadly used in the biological classification of species. Other ways to differentiate species are based on their anatomy, behavior, genetics, or evolutionary history. There is no single undisputed approach on how to classify two animals or plants as species or sub-species.

Genetics as Powerful and Precise Tool

The latest addition to this array of tools is genetics. “The amount of difference in DNA is a test of the difference between one species and another – and thus how closely or distantly related they are”. We could compare an entire genome, or just a small part of it, like mitochondrial DNA.  

Mitochondrial DNA. Image credit: File:Mitochondrial DNA lg.jpg

Mitochondrial DNA (mtDNA) in animals consist of approximately 16,500 bases (nucleotides). Humans and chimpanzees are different species for sure. The difference between chimpanzees and modern humans is approximately 1,462 mtDNA bases. That is 8.9% of mtDNA bases.

      Members of the genus Homo are much closer to each other. The difference between Sapiens and Denisovans DNA is 385 mtDNA bases. The difference between Sapiens and Neanderthals DNA is 202 mtDNA bases. In other words, Sapiens DNA differs from Denisovans and Neanderthals DNA respectfully by 2.3% and 1.2% of mtDNA bases.

At the same time, the typical difference in mitochondrial DNA within a species, including humans, is just 0.1%.

     Could we say that if somebody has mitochondrial DNA, which differs from an average Sapiens by more than 1% and less than 8%, then this “somebody” is a human, but non-Sapiens specie? Well, it is not that simple. However, at least for genus Homo, and, more broadly, in hominids family we are getting closer to identifying different species by quantitative measures based on DNA.

Ready to Retire an Old Tool

There is a conflict between various ways on how to identify different species. On one hand, it is well known now that Homo Sapiens, Neanderthals, Denisovans, and other members of the genus Homo interbred with each other multiple times in history. On the other hand, the mitochondrial DNA of our closest cousin, Neanderthals, has a 1 percent difference from Sapiens. And we know that, on average, a genetic diversity of mitochondrial DNA within different individuals of the same species less than 0.1 %, which is 10 times less than the difference between Sapiens and Neanderthals.

      For genus Homo, scientists lately decided to rely on genetics for species differentiation. It is important to have it in mind going forward.

Second Look at Our History

Once upon Carl Linnaeus’ time people “knew” we are humans and the history of mankind is our history. Then we understood that we, humans, are separate species and we called ourselves Homo Sapiens. We also called the genus to which we belong a genus Homo. At that time, we still had no idea that there were other non-Sapiens members in the genus Homo.    

Time passes. First Neanderthals remains were found and researched. The conclusion was that Neanderthal is so different from us, Sapiens, that we should call them non-Sapiens and include them in genus Homo as separate species.

Depiction of La Chapelle-aux-Saints Neandertals made in 1909. Image credit: http://www.talkorigins.org/faqs/homs/savage.html

  It was assumed Sapiens and Neanderthals could not produce healthy offspring even if they could mate. 

     Time passes again. We know now that there were at least 8 – 9 different now-extinct non-Sapiens species in the genus Homo. And we know much more about various waves of those humans’ migrations from Africa.

Then genetics entered the scene. Genomes of Neanderthals and Sapiens were sequenced. We see now how close Neanderthals were to Sapiens.

Neanderthal Child by Tom Björklund. Image credit: https://commons.wikimedia.org/wiki/File:Neanderthal_child.png

We got precise scientific data about what happened tens or hundreds of thousands of years ago. The amazing new facts are revealed. Most, if not all types of human species, mated with each other when it was possible. Most people of the current Sapiens population have some percentage of Neanderthals or Denisovans DNAs. An inability to mate as a differentiator between various members of the genus Homo is dead.

Last Chapter of This Story

     The DNA difference is used now as the differentiator for the human species. We understand that an overall history and prehistory of mankind includes the history of all humans, Sapiens and non-Sapiens, ever existed on this planet. If we want to limit our history or prehistory only to Sapiens – then we need to use a timeframe when all non-Sapiens humans got extinct. Or, we should find out other, at least equally powerful ways, to separate those histories.

    During just a few last centuries our understanding of who we are and what is our history and its timeframe have changed fundamentally. Of course, you will notice that if you look beyond just the last five thousand years of history of humanity.

In the next post, we will find out if Sapiens are splitting into different sub-species.

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By victortorvich

I'm a scientist and a writer. My upcoming book is “Subsurface History of Humanity”.

4 replies on “To Be Species or Not”

All the mating makes me wonder how Homo sapiens, Neanderthals, and Denisovans, saw each other when they met. Whether or not they recognized each other as people (Homo sapiens aren’t good about even recognizing other ethnicities as full persons), they certainly would have perceived they weren’t like other animals.

John Hawks noted that our picture of human evolution is built on a smattering of evidence, so it doesn’t take much to alter that picture. I suspect we still have a lot of changes in that picture to come.

Thank you, Mike.
Yes. History is not carved in stone. That is especially true when we are looking at events, for which we do not have written records. I hope it is clear that this is one of the main underlying topics in my series of history of humanity blog posts.

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