Genetic Origins

Genetic Origins

As an aspiring anthropology popularizer I frequently encounter people who ask me why anthropology needs to be popularized. Anthropologists collect data, analyze the results, publish in academic journals, and eventually some of their findings will be used to update undergraduate textbooks. What is the point of trying to engage with the rest of the world?

Well, to me this is a very simple question to answer. Anthropology needs to be popularized because it is the science of our species and the knowledge from this area of scientific inquiry can help people understand what it means to be human. Unfortunately, the best minds of our field don’t actually popularize anthropology very well; if at all. I think Barbara King did a great job of pointing this out at the end of one of her recent NPR 13.7 blog posts.

So whenever I am in a position to communicate the most interesting and insightful aspects of my specific subfield, evolutionary anthropology, I jump at the opportunity. Luckily, I had a chance to listen and discuss recent discoveries from the frontier of molecular anthropology last night with one of the top professors in the field: Esteban Parra. I want to share some of these findings with you.

But first, what is molecular anthropology?

Molecular anthropology is the study of our species at the genetic scale. Insights from this field of inquiry can help us better understand the connections between ancient and modern populations. I believe trying to understand our connection to the past is something that humans have always done. And like many things we do, it appears to be uniquely human. It doesn’t matter how many words in sign language you teach a chimpanzee, they never ask “where did I come from?”

What do we know about our origins?

But luckily for all of us endlessly curious humans, molecular anthropologists have uncovered a great deal about our origins. And the technologies utilized by molecular anthropologists have been advancing at such a remarkably quick pace, that new discoveries are made every year that reveal insights into our relationship to the past. For me, learning about our origins can be as humbling as learning about quantum superposition, mitosis, or the expansion of the universe. So I hope you get as much out of this article as I got out of my conversation last night.

There are 38 million variants of difference within our species

The Human Genome Project was completed in 2000. But that was just the beginning. After sequencing one human genome we had no idea what type of genetic variation there was within our species. However, the 1000 Genomes Project is the first phase of molecular anthropological research that is attempting to answer that question. Current data indicates that of the 3 billion base pairs within the human genome, there are 38 million variants of difference within our species. Surprisingly, most of these variants of difference only appear in one or two people, or one or two populations.

1000 Genomes Project

This was originally perplexing to researchers. Why were most unique variants restricted to such small scales within our species? The answer: our demographic explosion.

Population explosion

Within the past two centuries our population has ballooned from one billion to seven billion. And with population growth comes an increase in mutation rate. However, all of the unique genetic variants that have resulted from this increased mutation rate have had no opportunity to spread throughout our species because they have all arisen within the past few generations. As a result, rare variants are largely constrained to continents, and most of those variants are constrained to specific populations, and even individuals. This should create an even greater impetus to expand the 1000 Genomes Project and incorporate all human populations in the analysis.

Humans are much less genetically diverse than the great apes

great ape and human genetic diversity

I recently wrote an article about how understanding chimpanzee genetic diversity may be a key to help conservation efforts throughout Africa. As it turns out, chimpanzees, bonobos, gorillas, and orangutans are all more genetically diverse species than humans. This is an incredible discovery. How can a population as phenotypically diverse as human beings with seven billion individuals, be more genetically homogenous than species with low phenotypic diversity and fewer than a million individuals? This is something I’ve touched on in the past, and it has to do with a) the age of our lineage and b) our rate of migration.

Our species is actually young on evolutionary timescales. Anatomically modern humans emerged in East Africa between 150,000-200,000 years ago. Chimpanzees, bonobos, gorillas, and orangutans have all been in existence for hundreds of thousands of years longer than that. As a consequence, we are far less diverse in comparison. Another important factor, is that humans migrate quickly when compared to the great apes. Consequently our rate of gene flow between populations is much higher than between populations of great apes. Gene flow always decreases diversity and genetically homogenizes a species, and as it turns out, our species is a great case example of that effect.

Founder effect dictates degrees of human genetic diversity within our species

Founder effect and human genetic diversity Source: The great human expansion

Founder effects generate genetic bottlenecks. In order to imagine this, picture that you have one large population, and a few individuals within that population separate from the mother group and start a new population that interacts only indirectly with the parent population. The population that separated from the parent population will have relatively lower genetic diversity than the mother population because a few individuals will now represent the birth of a new, relatively distinct gene pool. With enough time and a high degree of isolation, this is how new species usually form. However, the founder effect for humans has been so rapid that it just created decreased genetic diversity for each founding group. As a result a decrease in genetic diversity is correlated with temporal patterns of continent-sized human migration events. The following is a representation of most genetic diversity to least genetic diversity by continent:

Africa > Europe/Middle East/South Asia > East Asia > Oceania > Americas

Molecular anthropologists already knew that the oldest human lineages and the most diverse human lineages were in Africa. This made intuitive sense because East Africa was the birthplace for our species. However, it was interesting to find that subsequent founder effects continually decreased genetic diversity throughout our species.

Neanderthals and Denisovans were technically modern humans

Neanderthals and modern humans

We’ve all heard of Neanderthals, and we have all heard narratives explaining the likely relationships between our species and theirs. But recent insights have indicated Neanderthals contributed to the modern human gene pool. In fact, all non-Africans share approximately 1-4% of their genes with Neanderthals. That is not much, but it is evidence of interbreeding. So were they really a separate species? Well, that depends on your classification scheme. But since we interbred and produced fertile offspring, imagining them as a distinct population may be superficial. This could be massively relevant information in regards to current debates stimulated by geneticist George Church.

Interestingly, these genetic insights also give us insight in terms of where interbreeding occurred. Since Neanderthal genes are equally distributed among all non-Africans, interbreeding likely occurred in the Middle East, not Europe. If interbreeding occurred in Europe, we should expect contemporary Europeans to have higher frequencies of Neanderthal genes than other non-African populations. However, since the distribution is even, the first human migrants out of Africa likely interbred with Neanderthals, and carried those genes with them into Asia, Australia, Europe and the Americas.

Denisovans are a lesser known hominid. They were recently discovered and their genome was sequenced last year. We know that they lived in Siberia (but we don’t know their full range). We also know that they are sister taxa to the Neanderthals, but they are still genetically unique enough to have been classified as a different species. However, like Neanderthals, we know that modern humans had sex and produced fertile offspring with them. Interestingly, Denisovan genetic variants appear at higher frequencies within Australian aborigine populations. As a result of this finding, it is likely that the first migrants to journey through Asia to Australia interbred with Denisovans along the way. Similarly to our situation with understanding Neanderthal taxonomic classification, these results indicate that Denisovans were also modern humans, that were just as ancient as the Neanderthal lineage.

Understanding what it means to be human

So is their a need to popularize anthropology? I hope this article was a brief example of the benefits of communicating anthropological knowledge to a wider audience. Insights in all subfields provide us with an opportunity to contemplate our past, and gain a better understanding of our current world. In the future, anthropological research should continue to uncover clues from the past that can help us to piece together our origin. If learning about human genetics was new, I hope that it piqued your curiosity about where we came from and about what we can learn from anthropological inquiry.

 
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