A person named Brit Nicholson published on Medium a sensible overview of the controversy over how long ago was the Most Recent Common Ancestor of all living humans.
The first thing to keep in mind is that having any one single ancestor long ago is a largely symbolic discovery.
Do all humans share a common ancestor from a few thousand years or a few hundred thousand years ago?
Brit Nicholson
Jun 12, 2019
… In his fascinating book “A Brief History of Everyone Who Ever Lived,” Adam Rutherford makes the astounding claim that all humans alive today share a most recent common ancestor (MRCA) from somewhere between 3,400 and 3,600 years ago, i.e. at least one person lived during that time who is the ancestor of every person alive today. Rutherford even notes the frowns of disbelief on audience members’ faces when he includes this claim in his speeches. Indeed, it flies in the face of our intuition.
One of the first things that come to mind upon hearing Rutherford’s claim is that it would only take one isolated population on an island somewhere for it to be wrong. Then again, it would only take one immigrant on a boat to the island two thousand years ago to have easily become an ancestor of everyone on that island today, or one conquistador who left descendants in remote areas of the Amazon. And they may not have left any genes that survive to the present day, which would make it impossible to prove or disprove the claim. However, it seems more likely that some tribe has remained untouched than that every tribe in the world has had at least one recent immigrant.
… The main theme of Rutherford’s book is that humans have always been enthusiastic about both migration and reproduction — that no population stays in the same place, unmixed, for very long.
Adam Rutherford likes to portray his theory of a recent MRCA as a beautiful fact, but it actually seems kind of horrifying if you think of the sad fates of the Tasmanians, which is all that makes his argument plausible. The last full-blooded Tasmanian is said to have died in 1876.
People could walk from Australia to Tasmania until about 8,000 years ago when warmer weather raised sea levels. Now, the Bass Strait is about 160 miles wide, with rough seas. So, it’s unlikely that any Australian Aborigines got to Tasmania from 6000 BC onward until fairly recently.
But now the full-blooded Tasmanians are gone, so Rutherford’s theory is less implausible.
In another wonderful book, “Humans: Who We Are and How We Got Here,” David Reich similarly notes that our family “tree” is actually more like a lattice. …
Reich’s estimation of when our MRCA lived is wildly different than Rutherford’s. Part of the reason for that is that Reich’s estimation is based on genes, and the contributions of many of our ancestors disappear over the generations. Here’s an excerpt from Reich’s book:
Across chromosomes 1–22, the most recent shared ancestor for all present-day people ranges mostly between 5,000,000 and 1,000,000 years ago, and nowhere is it estimated to be more recent than about 320,000 years ago.
So the estimation is ~3,000 from Rutherford and ~300,000 from Reich.
One of these figures, given by two experts in human DNA science, is very wrong. How do we reconcile the difference?
In Rutherford’s book, he says that his figure comes from a model by the mathematician Joseph T. Chang in 2003. The mathematical model, first published in 1999, is truly elegant, but it only applies to closed populations with random mating. Chang lists multiple constraints for his model, including that it can’t be used for humans because of geographical and other barriers. The results of this model show us that MRCAs for most populations occur far more recently than anyone would expect, but even when allowing for migration within the model I believe that it doesn’t apply to all populations of humans.
The figures 3,400 to 3,600 are in fact from a paper by Douglas L. T. Rohde published in 2004 with Chang as a co-author. Part of Rohde’s computer simulation takes into account Chang’s probabilistic model from 1999. Rohde’s model was an excellent idea and was well executed, but it has a lot of constraints of its own. For example, the model divides the world into ten nodes that roughly correspond to continents. Migrants are swapped between continents in each generation. The amount of migration in the model is considered a conservative estimate, but in some cases in the real world there may have been no swapping of migrants.
I wonder when the first person born in Africa set foot in South America, or vice-versa? Columbus landed in Venezuela in 1498, and he might have had an African on board.
A more remote possibility is that some Carthaginian navigators went west of the Straits of Hercules and got blown to South America. (Berbers were living on the Canary Islands a few hundred miles out in the Atlantic when Europeans got there around 1300 AD.) Stories of castaways are inherently interesting, as Daniel DeFoe showed to his vast profit, so I’ll speculate a little on the subject.
This is noted in the paper, and examples are given of populations that remained isolated for long periods of time, such as Tasmanians, but it’s claimed that there are no longer any populations that are known to remain isolated to the present day.
Nicholson brings up another candidate: North Sentinel in the Andaman Islands of the Indian Ocean, where the natives have killed at least intruders in this century.
Other locations where humans lived in isolation from people from other continents for longer than 4,000 years might include Tierra del Fuego at the extreme end of South America, although I don’t know whether there are any pure blooded indigenous people left of the kind Darwin met in the 1830s. (There is an old lady on Tierra del Fuego still alive who is said to be the last full-blooded member of her tribe, although she looks fairly European to me.)
There are two main ways that people on different continents could wind up with common ancestors: diffusion versus long leaps.
For example, the Pericú people of Cabo San Lucas in Baja California, who were a distinct group into the 18th Century before they were civilized into history by the Jesuits, are said to have looked and behaved quite different from American Indians (they were still using the atlatl and dart instead of the bow and arrow in the 1600s), leading to speculation that either they “were either trans-Pacific immigrants or remnants of some of the New World’s earliest colonizers.”
Two interesting questions is how often did navigators get blown by ill winds all the way to another continent? And how many of them were afforded women when they got there?
We know a little bit about Japanese sailors who washed up in North America. Cassandra Tate writes:
According to historian Frederik L. Schodt, at least 34 Japanese sailors reached the shores of North America or Mexico on disabled ships between 1806 and 1852.
There are winds frequently blowing from Japan to North America (which is how the Japanese during WWII tried to set forest fires in the U.S. by sending fire balloons across the Pacific). And 19th Century Japan had more advanced technology than most of human history before then, so their odds of surviving a long drift at sea were better.
One of the best known cases involved the Tokujomaru, which ran aground near Santa Barbara, California, in 1813, with three survivors out of a crew of 14.
These castaways were treated kindly by Westerners and two of them made it home to Japan. I presume washed-up sailors could generally make themselves useful on return voyages, so Westerners had evolved pretty humane customs for helping castaways.
But others weren’t so lucky. In the 1830s, three Japanese castaways, out of a crew of similar size (most of whom died of scurvy), were washed up on the Olympic Peninsula of the future state of Washington. They were immediately seized by Makah Indians:
Then they escorted the three hapless seafarers inland to a Makah village and held them there as slaves (a commonplace practice among coastal tribes at the time).
Fortunately, the Hudson Bay Company rescued the Japanese seafarers and set about trying to get the xenophobic Tokugawa shogunate to agree to exchange of castaways. But they were not allowed back in Japan and wound up in Macau, where one became a successful translator and married first a British woman and then a Malay woman.
So it’s hard to say from these documented cases how many prehistoric castaways were welcomed into communities on the far side of the ocean. Most would have been fishermen or sailors rather than female passengers. And most would have been in terrible shape from scurvy when they arrived.
