Great Ape and Human Genetic Diversity

Last week I wrote about what molecular anthropologists know about our genetic origins.  Within that post I discussed human genetic variation, great ape genetic variation, the founder effect, and hominid genetics.  However, I feel as though I didn’t spend enough time explaining one mind-bending aspect of these discoveries: genetic variation between humans and great apes.

All great apes are more genetically diverse than our species: Homo sapiens.  Recent genetic studies by molecular anthropologists have revealed that there are only 38 million unique genetic variants among the 3 billion base pair sequences within our species genome.  Although 38 million unique variants may seem like a lot, it is actually a very small fraction of our genome.  All the great apes are far more dissimilar than humans.  We appear to be a homogenous group!

But this begs the question: why are we so similar?

On further reflection it becomes very perplexing.  After all, there are seven billion humans on Earth!  That’s a lot of Homo sapiens.  And we don’t all look similar; there is incredible phenotypic diversity within our species.  There are people with all different shades of skin colour, hair texture, height, weight, shape, and size.

In contrast, chimpanzees, bonobos, gorillas, and orangutans all have relatively small populations.  In the wild, there are <250,000 chimpanzees, <110,000 gorillas, <60,000 orangutans, and <50,000 bonobos.  Furthermore, there appears to be relatively little phenotypic diversity among these species.  It is difficult to tell different subspecies of great ape apart, even for experts!

So why are all the great apes more genetically diverse than us?  Shouldn’t a species with 7 billion phenotypically diverse individuals be more genetically diverse than species with less than a million individuals that all look relatively similar?  Apparently not; molecular anthropology is teaching us that our differences are genetically superficial.  And there are two main reasons why we are so genetically similar: a) the age of our species and b) our rate of migration.

Our species is actually very young.  Anatomically modern humans have only been on the planet for 150,000-200,000 thousand years.  That may seem like a really, really long time.  But in fact, it is not that long on evolutionary timescales.  All species of great ape have existed for hundreds of thousands of years more than our species.  We are young apes!

But why does being a young species have anything to do with genetic diversity?  Well, the number of unique mutations that could have accumulated within our lineage is largely dependent on how long we have existed.  Most human lineages are 150,000 years or younger, which isn’t very much time for mutations to accumulate between or within populations.

A second factor causing our low genetic diversity is our migration rate.  Humans move quickly, and consequently, so do our genes.  Our migration speed is much faster than any other great ape species, and it has been that way since we emerged in East Africa 150,000 thousand years ago.  We have always been mobile, exploring new landscapes, and continents.  In contemporary times, this rate of migration has only increased.  In contrast, great apes have static home ranges on a continental scale; they don’t move around too much.

But again, why does a quicker migration rate have anything to do with the level genetic diversity?  It all has to do with gene flow.  Gene flow is the transfer of genes and/or alleles from one population to another.  And gene flow always decreases diversity because it homogenizes the gene pool of otherwise genetically distinct populations.

As a result, chimpanzees, bonobos, gorillas, and orangutans have existed for a very long time, in more or less the same areas.  They have had ample time to accumulate novel genetic mutations between and within their populations.  And because they very rarely interact and mate with distant groups, their rate of gene flow is low; which increases genetic diversity.

So what does this all mean for us humans?  I think it means that regardless of how different we all look our differences are largely superficial.  Our species is young and increasingly interconnected.  In the future, these facts about our biology should be reason for hope and optimism.  Our genetic heritage unites us, it does not divide us.

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About Cadell Last
Hello. I'm probably drinking coffee and reading.

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