How the human MRCA possibly have lived just 3,000 years ago?

[bluemoonKY]

https://en.wikipedia.org/wiki/Most_recent_common_ancestor

I was reading about the human most recent common ancestor (MRCA) at the above link at Wikipedia. According to this article at Wikipedia, there have been common human ancestors to all humans alive today. This sounds far-fetched to me. But what seems far more far-fetched to me than that is that the most recent common human ancestor may have lived just 3,000 years ago.

From Wikipedia: "The age of the human MRCA is unknown. It is necessarily younger than the age of both Y-MRCA and mt-MRCA, estimated at around 200,000 years, and it may be as recent as some 3,000 years ago."

As of 3,000 years ago in 983 B.C., there were already millions of humans on all the continents of the Earth except for Antartica. I'm assuming that the human MRCA lived in Africa. But my point is just as valid if the human MRCA did not live in Africa. Let's say that the human MRCA lived in Africa 3,000 years ago. How can this African human in 983 B.C. be the ancestor of the Native Americans? For this human MRCA in 983 B.C. to be the human MRCA, this human would have to be the ancestor of the Native Americans. But the Native Americans have been in America for over 10,000 years. How can this human MRCA in 983 B.C. possibly be the ancestor of Native Americans when the Native Americans migrated to the Americans before the human MRCA in 983 B.C. was even born?

What am I missing here?

I know that the Wikipedia link does not say that the human MRCA definitely was around only 3,000 years ago. I know that the Wikipedia link says that the human MRCA only say that the human MRCA possibly lived just 3,000 years ago. But how is it even possible?

 

[mfb]

It is not the ancestor of all humans who have ever lived, it is an ancestor of all humans living today.
It should be obvious that such a person exists - otherwise humans would have evolved twice independently in exactly the same way and the two lineages would have been perfectly separated the whole time. That doesn’t happen.

Humans reproduce sexually. With a constant population an average person will have two children, four grandchildren, eight grand-grandchildren and so on. Over time, there are just three options for a given person:
* All descendants of the person die
* The world has two or more populations that are strictly separated and never have children across this border.
* Everyone living in the world is a descendant of the person.

The second case doesn’t happen.

 

[Drakkith]

For reference, the paper that gives the 3,000 YA number can be read in full at the following link: https://www.nature.com/articles/nature02842

Edit: Apparently that's a paywall for most people.

 

[bluemoonKY]

It is not the ancestor of all humans who have ever lived, it is an ancestor of all humans living today.
It should be obvious that such a person exists - otherwise humans would have evolved twice independently in exactly the same way and the two lineages would have been perfectly separated the whole time. That doesn’t happen.
.

mfb, before I made the OP, I was thinking that one common ancestor of all humans alive today was implausible because I thought that large masses of lower primates evolved into humans. I thought that large masses of lower primates evolved simultaneuously with each other. However, perhaps the first human evolved due to a random mutation of DNA. If the first human (who would be more like a lower primate than any human who ever lived) who ever lived was human due to a random mutation of his or her DNA, then all humans would indeed be descended from that one human. In any case, the idea of all humans alive today coming from one common ancestor really is not the primary focus of the OP. The primary focus of the OP is the assertion that the human MRCA was alive only 3,000 years ago. You just totally ignored that part of the OP. You know why I think you ignored it? You cannot explain it yourself.mfb, please address this part of the OP:

As of 3,000 years ago in 983 B.C., there were already millions of humans on all the continents of the Earth except for Antartica. I'm assuming that the human MRCA lived in Africa. But my point is just as valid if the human MRCA did not live in Africa. Let's say that the human MRCA lived in Africa 3,000 years ago. How can this African human in 983 B.C. be the ancestor of the Native Americans? For this human MRCA in 983 B.C. to be the human MRCA, this human would have to be the ancestor of the Native Americans. But the Native Americans have been in America for over 10,000 years. How can this human MRCA in 983 B.C. possibly be the ancestor of Native Americans when the Native Americans migrated to the Americans before the human MRCA in 983 B.C. was even born?

 

[jim mcnamara]

@Drakkith Paywall. The model discussed is based on a completely random mating of humans. Taken literally it means you have equal probability of having children with somebody from Katchatcka, or Lima Peru, or Teunis Algeria - back several thousand years ago. I think you can see what this means: it is impossibly impractical, even today.

Mitochondrial Eve is dated to more than 200K years ago. One possible MRCA in Africa.

y-MRCA is usually dated for a population. NB: the word population. Population is defined as a small number of freely interbreeding humans, with some limited gene flow into the population. Not the count of humans in Clay County Alabama in 2010.

Also the Y chromosome has a fairly high rate of conformity - there is generally no crossing over, so the DNA there does not change, in contrast with mtDNA which has a higher rate of change due to mutation - again no crossing over since this is somatic inheritance.

@bluemoonKY

So, how come 3000 years? The Y chromosome is very sensitive to migration so founder effect can have big impact.

[contrived example]
Founder Effect Example in humans: In the SW US there are tribes of Indians, among them are Keres speaking tribes. If you marry out of the tribe you do not live in the village. So, obviously they frown on it. We know they once were all up near Cochiti NM as early as 1200CE. In the past 600 years or so, small groups broke off from other larger groups, went away angry and took a kiva with them to found a new "tribe". Still speaking Keres.

Examples: Cochiti -> (Fork) Acoma (Fork) Santo Domingo -> San Felipe (two forks here) one to Santa Ana I -> one later to Santa Ana II (not US Gov't recognized AFAIK). Based on some DNA samples you would see exactly the pattern I described: groups splitting off and certain Y gene frequencies becoming higher since the founding genetic diversity diversity was reduced. Maybe even unique DNA. This is not part of a publication AFAIK.
[/contrived example]

This is a great link for Founder Effect (Bottleneck): https://evolution.berkeley.edu/evolibrary/article/bottlenecks_01

Now imagine that all the villages except Santa Ana II died out from smallpox. We could not very easily do the trace back. We would conclude that the yMRCA was in Santa Ana II and came there in the 1500's. Nobody anywhere else has those unique yDNA genes/gene frequencies. Hence it is possible to get small numbers of years for some specific yDNA pattern labelled as the start in small founding population sizes. Even if there were hundreds of thousands of living descendants.

This is a contrived example, but you get the point. Population based yDNA is biased to founders of small populations.

 

[bluemoonKY]

jim McNamara, you failed to answer the question.

 

[Drakkith]

I thought that large masses of lower primates evolved simultaneuously with each other. However, perhaps the first human evolved due to a random mutation of DNA. If the first human (who would be more like a lower primate than any human who ever lived) who ever lived was human due to a random mutation of his or her DNA, then all humans would indeed be descended from that one human. I

Humans evolved via a large number of mutations in their DNA accumulated over several million years since our common ancestral population split from those of chimpanzees and/or bonobos. If you were able to travel backwards in time, there is no single point in time you could go to and point to and say, "There! That's the first human!" Any such choice would be entirely arbitrary. Instead our ancestors gradually changed into ourselves as time passed.

Also note that the MRCA is not more primitive than any human that has ever lived. That would be the last common ancestor between ourselves and chimps/bonobos, or their first descendants (or something further down the line depending on how you choose to define "humans").

You just totally ignored that part of the OP.

He did not. I had just started typing up a post explaining the same thing when MFB's post popped up. The problem is that you don't understand what the MRCA means. See below.

How can this human MRCA in 983 B.C. possibly be the ancestor of Native Americans when the Native Americans migrated to the Americans before the human MRCA in 983 B.C. was even born?

Because the MRCA is one of the ancestors of current, living Native Americans. They were not an ancestor to Native Americans living thousands of years ago (though they could be if they actually lived prior to these ancient Native Americans). The MRCA is an ancestor common to all current living humans. They became this because their children, grandchildren, and other descendants spread out throughout the world over time. Eventually all living humans were able to trace part of their lineage back to this one person. It's not that we don't have other ancestors, or even other common ancestors, but the MRCA is the most recent common ancestor.

Note that the recent figure of about 3,000 YA is based on a very speculative model that makes a large number of simplifications, so don't take that number as being accepted as likely by the scientific community. The paper even mentions that it doesn't claim that the MRCA actually lived this recent, only that the paper's models support the idea that they could have.

@Drakkith Paywall.

Really? I wonder why I'm able to access it then...
Must be because I'm using a computer on-campus right now.

 

[bluemoonKY]

Because the MRCA is one of the ancestors of current, living Native Americans. They were not an ancestor to Native Americans living thousands of years ago (though they could be if they actually lived prior to these ancient Native Americans). The MRCA is an ancestor common to all current living humans. They became this because their children, grandchildren, and other descendants spread out throughout the world over time. Eventually all living humans were able to trace part of their lineage back to this one person. It's not that we don't have other ancestors, or even other common ancestors, but the MRCA is the most recent common ancestor.

I understand the implications of the MRCA being the ancestors of current, living Native Americans as opposed to the MRCA being the ancestor of all the Native Americans three thousand years ago. However, I don't think that explains how the MRCA could be as recent as 3,000 years ago.

There were millions of Native Americans in America in 983 B.C. I know that most of the Native American population in North America was killed by the diseases that the white man brought. However, there has been no point in time from 983 BC to today in which there were not millions of people of Native American descent. The population bottleneck after 1492 was not that small. How is it possible that the descendants of the human MRCA were so numerous as to breed with at least one of the ancestors of all those millions of people of Native American descent between 983 BC and today? I just don't see how there could be so much darn population overlap in 3,000 years. That's just approximately 120 generations.

 

[bluemoonKY]

Humans evolved via a large number of mutations in their DNA accumulated over several million years since our common ancestral population split from those of chimpanzees and/or bonobos. If you were able to travel backwards in time, there is no single point in time you could go to and point to and say, "There! That's the first human!" Any such choice would be entirely arbitrary. Instead our ancestors gradually changed into ourselves as time passed.

If there was just one single first human who evolved by random mutation, and if then all other humans were created by this first human breeding with other lower primates, then I can understand how we can definitely know that all humans are descended from at least one common human ancestor.

However, you dispute my understanding of this in your quote above. If what you are saying is true, then how can we definitely know that there is a common HUMAN ancestor of all humans alive today? I concede that if your understanding of the evolution of humans is correct, a common human ancestor is likely. But how can we definitely know that there is a common HUMAN ancestor of all humans alive today?

 

[Drakkith]

There were millions of Native Americans in America in 983 B.C. I know that most of the Native American population in North America was killed by the diseases that the white man brought. However, there has been no point in time from 983 BC to today in which there were not millions of people of Native American descent. The population bottleneck after 1492 was not that small. How is it possible that the descendants of the human MRCA were so numerous as to breed with at least one of the ancestors of all those millions of people of Native American descent between 983 BC and today? I just don't see how there could be so much darn population overlap in 3,000 years. That's just approximately 120 generations.

It only takes as little as one interbreeding event to introduce the MRCA's lineage to that population. Once introduced, any child between someone of the MRCA's lineage and someone who's not results in a new generation of the MRCA's lineage. This lineage can rapidly spread throughout a population, especially a small population where mate choice is limited and most people are only a handful of generations removed from each other. And that's only from a single initial interbreeding event. We know for fact that the many different small population groups of Native Americans readily interbred with each other and even those of European descent, further reducing the time needed to spread the MRCA's lineage.

If there was just one single first human who evolved by random mutation, and if then all other humans were created by this first human breeding with other lower primates, then I can understand how we can definitely know that all humans are descended from at least one common human ancestor.

However, you dispute my understanding of this in your quote above. If what you are saying is true, then how can we definitely know that there is a common HUMAN ancestor of all humans alive today?

I don't know. Someone else will have to explain that. I'll try to look into it and get back to you though.

 

[jim mcnamara]

jim McNamara, you failed to answer the question.

@bluemoonKY
That's interesting. I did PhD research in Population Biology, which is exactly this content. The correct answer is: yMRCA does not work the way you think it does. It works the way it "wants" to. I'm going to let it go because I am not helping anything.

A point: if someone does not phrase a response in the words you want, do not assume you did not get a decent answer. Simply say 'Can you say that another way?'

 

[bluemoonKY]

@bluemoonKY
That's interesting. I did PhD research in Population Biology, which is exactly this content. The correct answer is: yMRCA does not work the way you think it does. It works the way it "wants" to. I'm going to let it go because I am not helping anything.

When you say yMRCA, do you mean the Y-chromosome MRCA ? Is this the same as Y-chromosome Adam?

Can you say that another way?

 

[mfb]

@bluemoonKY: You seemed to have missed the second part of my post, complaining that a part of the post is not addressing all questions asked is a bit odd.
We all share billions of universal common ancestors. Every one of the earliest humans (no matter where exactly you set the arbitrary cutoff) is either an ancestor of all of us or of none of us.

Imagine a single European getting a child with a native American, with the child staying with the native population. On average, we expect it to have two children, four grandchildren, ... and 10 generations later we have around 1000 descendants. Probably a bit lower as some descendants will get children with other descendants.

To have a very recent common ancestor all you need are a few people traveling and getting children with other populations, and then a few hundred years to spread the ancestor relation in the population.

 

[bluemoonKY]

@bluemoonKY: You seemed to have missed the second part of my post, complaining that a part of the post is not addressing all questions asked is a bit odd.
We all share billions of universal common ancestors.

I agree that we definitely know that we all share billions of universal common ancestors if you don't limit the word ancestors to mean human ancestors. How do I know this? Because we know from DNA that all life is descended from a common ancestor. We have common ancestors with an oak tree.

Every one of the earliest humans (no matter where exactly you set the arbitrary cutoff) is either an ancestor of all of us or of none of us.

Nobody has proven that yet.

Imagine a single European getting a child with a native American, with the child staying with the native population. On average, we expect it to have two children, four grandchildren, ... and 10 generations later we have around 1000 descendants. Probably a bit lower as some descendants will get children with other descendants.

This does not prove your case either.

To have a very recent common ancestor all you need are a few people traveling and getting children with other populations, and then a few hundred years to spread the ancestor relation in the population.

How do you know that we all have a common human ancestor?

 

[mfb]

I agree that we definitely know that we all share billions of universal common ancestors if you don't limit the word ancestors to mean human ancestors. How do I know this? Because we know from DNA that all life is descended from a common ancestor. We have common ancestors with an oak tree.

I’m only talking about human ancestors. We all share billions of human ancestors.

Nobody has proven that yet.

Proof are mathematics. It has been shown beyond reasonable doubt, and the reasons why have been explained in this thread.

This does not prove your case either.

I’m trying to help you to understand. I thought that’s the point of this thread.

How do you know that we all have a common human ancestor?

See above, especially my first post.

 

[Buzz Bloom]

The model discussed is based on a completely random mating of humans.

Hi Jim:

Just wondering. Does anybody take seriously that a model based on random mating can actually make a plausible estimate of a date for the MRCA?
The abstract of the article used the phrase "Monte Carlo simulations."

A more elaborate second model, designed to capture historical population dynamics in a more realistic way, is analyzed computationally through Monte Carlo simulations.​

With out access to the article itself, I am not sure how this simulation was done, but I am guessing that it failed to realistically take into account how an MRCA from the European-Asian-African mega-continent from 3000 years ago could possible had descendants that interbred with many of the aboriginal tribes in Brazil.

From Wikipedia https://en.wikipedia.org/wiki/Indigenous_peoples_in_Brazil
On January 18, 2007, FUNAI reported that it had confirmed the presence of 67 different uncontacted tribes in Brazil, up from 40 in 2005.​

Regards,
Buzz

 

[Ygggdrasil]

From the associated News and Views article that accompanied the Rohde et al. paper:

The authors carried out simulations based on several scenarios, incorporating different degrees of population growth and different degrees of isolation of subpopulations, with occasional migration linking these subpopulations. The authors' first model is relatively simple and includes up to ten large subpopulations, which exchange just one pair of migrants per generation. In one set of estimates based on this model, the mean time back to the universal ancestor is 2,300 years (76 generations, assuming a generation time of a bit less than 30 years) and to the identical ancestors it is 5,000 years (169 generations) — the time of Aristotle and the first pyramids, respectively. The latter date is especially startling: had you entered any village on Earth in around 3,000 BC, the first person you would have met would probably have been your ancestor! A considerably more detailed model, which describes population density within continents, the opening of ports and more, does not change these estimates much.

The main weakness in the models comes from migration. As the authors point out, if one region is totally isolated (something that they do not simulate), with no migrants connecting it to other subpopulations, then the universal ancestor must logically have lived before the period of isolation began. Only after that period ends would the dates for the universal ancestor become less distant. Because of the effects of isolation, had we been living in 1700, say, and tried to work out when our universal and identical ancestors lived, the answers would have been further back in time than the answers we obtain now. Tasmania, for instance, was conceivably completely isolated at the time, and probably had been for millennia; this would therefore have pushed back the dates for universal and identical ancestry. So uncertainties about population structure introduce uncertainty into the proposed dates.

https://www.nature.com/articles/nature02842

The main point of the Rohde et al. paper is that relatively low migration rates would promote sufficient exchange of genetic material between populations to move the date of the MRCA to much more recent than one might think. The exact date is still very much up to debate, however, and needs experimental verification (as the Rohde et al paper is purely theoretical and does not use any actual observational data to back up its claims).

 

[Drakkith]

Just wondering. Does anybody take seriously that a model based on random mating can actually make a plausible estimate of a date for the MRCA?

I can't say I understand the details of what "random mating" means in terms of this method of modeling populations, but I trust that it's accurate enough for their purposes.