The Out of Africa Hypothesis

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Podcast Transcript

Several hundred thousand years ago, human beings walked out of Africa. 

What has been a subject of debate amongst anthropologists is why it happened, how it happened, and how many times it happened. 

The process by which homo sapiens left their land of origin to populate the rest of the world has been one of the fundamental questions in anthropology. 

Learn more about the Out of Africa hypothesis and the origins of humanity on this episode of Everything Everywhere Daily.

The Out of Africa hypothesis is perhaps more accurately called the Out of Africa hypotheses, as there are several different theories 

What all the various theories are in agreement on is that humans originated in Africa. 

To understand the theories, it is helpful to understand the origin of our understanding of early humans. 

In the 19th century, after Darwin wrote the Origin of Species, we didn’t have a lot of evidence for anything about early humans one way or another. 

Outside of modern humans, the only fossil evidence we had was from Neanderthal and Homo Erectus skeletons. That and comparative anatomy with other animals like chimpanzees and gorillas was it. 

Initially, it was assumed that the early hominids that were discovered were not related to humans at all. 

As you’ll see, the reason why theories of early humans have changed over time is that more and more evidence has been collected, primarily in the discovery of fossils, but also with improved techniques.

In the 20th century, more and more hominid fossils were discovered. In particular, Australopithecus fossils were discovered in the 20s and 30s, and Homo heidelbergensis skulls in the 1950s.

Slowly, anthropologists began to put the pieces together and assumed that these different species were in some way related. 

The theory which came out of this in the 20th century became known as the multiregional hypothesis of human evolution, also known as MRE. It was proposed in 1984 by Milford Wolpoff of the University of Michigan.

The multiregional hypothesis, in hindsight, seems sort of silly, but it was the working explanation for the rise of modern humans for several decades. 

It held that different hominid groups left Africa several hundred thousand years ago and evolved separately. The similarities with other groups which became human were due to the sharing of genes between groups that bordered each other and migrations. 

Under this theory, modern homo sapiens didn’t evolve in Africa per se, but it was a process that was undergone all over the Afro-Eurasian landmass.

The problem with this theory is severalfold. For starters, the odds that the same species would arise in different places and would be so similar is highly improbable. 

It also assumed that most of the hominid fossils which were found were, in one way or another, ancestors of humans. 

The MRE theory wasn’t the only one at the time.

Another theory, known as the Recent African Origin model, was proposed in 1974. This theory held that modern homo sapiens developed from one group in East Africa and then spread out of Africa rather recently, in the last 100,000 years or so. 

The thing which eventually resolved the debate to the satisfaction of most anthropologists was genetics. 

The advent of genetic testing allowed for a level of insight that was previously impossible by just looking at old bones. 

If the multiregional hypothesis was true, then humans should show signs of different ancient genes in different populations. 

If the Recent African Origin model were true, then we should only see a single set of older genes, and then we should see genetic mutations timed as groups spread away from Africa.

Here I should note that it is possible to approximately determine dates for genetic mutations over large populations due to a “molecular clock” that works inside DNA.  Mutations accumulate over time, and as techniques get better and better, it is possible to determine when genetic changes occurred. 

There have been tens of thousands of human genome sequences of  mitochondrial DNA which have been conducted on people all over the world. 

These genomes all conclusively point to the exact same thing: All humans came from a similar group that originated in Africa about 200,000 years ago, and all non-African populations split apart within the last 50-60,000 years.

If you remember back to my episode on Mitochondrial Eve, this is approximately around the time when she would have existed, about 155,000  years ago.

The genetic evidence was so overwhelming that the multiregional hypothesis is, for the most part, dead. 

What it also did was put to rest the idea that all of the early hominid fossils which have been discovered were ancestors of humans. What is showed is that many of them were just evolutionary dead ends. 

They weren’t necessarily our ancient grandparents, rather, they were our ancient cousins who moved to another city which we never heard from again. 

Genetic testing also unveiled several other surprising things. For starters, Neanderthals weren’t necessarily our ancestors, but they also weren’t that far away from homo sapiens genetically. 

There is evidence of neanderthal DNA in humans. The average human has about 2% neanderthal DNA, with some populations having as much as 3%. 

This neanderthal DNA is still making itself felt today. One particular genetic risk factor for COVID-19 was traced back to a fragment of neanderthal DNA which was found in European and South Asian populations but not nearly as much in East Asian and African populations. 

The confirmation of the recent African origin theory didn’t end the story of how modern humans spread around the world. 

If you look at a map, you can see the small bit of land in modern-day Egypt, which connects Africa to the Middle East. 

The question is, if one group could have entered this area, then why couldn’t others as well, especially considering that we are talking about a period of time of over 100,000 years? 

The answer is that is almost certainly what happened. The debate has shifted from the multiregional hypothesis vs. the recent African origin hypothesis to trying to nail down exactly when humans left Africa, how many times humans left Africa, and what routes they took out of Africa. 

It turns out that while the recent movement of humans out of Africa was the origin of modern homo sapiens it wasn’t the first out-of-Africa moment for related hominid species. 

Researchers have dubbed them Out of Africa I and Out of Africa II: Electric Boogaloo.

….OK, I added the Electric Boogaloo part, but that is what we should call it. 

Out of Africa I was the migration of early hominids from 500,000 to 1.8 million years ago. This would have included Home erectus, Homo heidelbergensis, and other groups, which eventually died out.

Out of Africa II was the migration of modern Homo sapiens out of Africa, starting with their appearance about 200,000 years ago. 

Out of Africa II, which is really the subject of everything I’ve been talking about so far, has become more nuanced. 

It appears there were multiple waves of Homo sapiens migrations out of Africa. 

Not all of the migrations were successful insofar as they established permanent populations. 

A migration around 275,000 years ago might have included a species closely related to Homo Sapiens, perhaps closer to Neanderthals.

Fossils found in a cave in Greece date back 210,000 years.  Modern human fossils found in the Arabian Peninsula and China date back about 85,000 years. 

These migrations might have resulted in populations that weren’t successful.

The migration, which was successful, occurred around 70,000 years ago. This took place sometime around the massive Mount Toba explosion in Indonesia, which will be a future episode.

This migration followed what was known as the southern route. They spread across Arabia, South Asia, East Asia, and Southeast Asia, including Indonesia, Papua New Guinea, and Australia, all of which would have been connected by land during the ice age. 

Humans probably entered Australia about 50,000 years ago. 

Europe and Northern Asia were one of the last parts of Eurasia to be populated. This was probably due to the climate. Homo sapiens were a tropical species and stuck to warmer areas until they developed the tools to survive in colder climates. 

Homo sapiens entered Europe around 40,000 years ago, where they also probably encountered Neanderthals and Denisovans. 

Humans didn’t cross the land bridge into the Americas until about 20,000 to 15,000 years ago. The current genetic evidence points to a single group that migrated from East Asia/Siberia, from which all native peoples in the Americas are descended. 

The story of human evolution and human migration is one that is constantly being updated as new evidence is found. 

Every year it seems that there are new fossil discoveries or new genetic research, which help to clarify the picture of how modern humans came to be.

They are constantly adding pieces to the puzzle of how humans originated and how we are all related to each other. 

The one thing that we can be sure of is that all of us, no matter who you are or where you are listening to this, have distant ancestors who came out of Africa. 

The Executive Producer of Everything Everywhere Daily is Charles Daniel.

The associate producers are Thor Thomsen and Peter Bennett.

Today’s review comes from listener KR687 over on Apple Podcasts in the United States They write:

Great podcast

This podcast has become my favorite. I have been catching up on older episodes and will be sad when I join the completion club. I love the way you cover each topic. I always learn something I didn’t know. Your podcast is good for the mind. Thank you Gary!

Thank you, KR687! Don’t be sad when you join the completionist club, be happy to know that you have joined one of the most elite groups in the world. 

Also, you can always go back and listen to everything again. That will get you upgraded to gold membership in the completionist club.

Remember, if you leave a review or send me a boostagram, you too can have it read on the show.