For example, how many generations removed are we from the last common ancestor of humans? From last common ancestor of us and chimpanzees? From the last common ancestor of all mammals? If you know, please keep going as far back as you like. Someone once told me that all humans are at most 50th cousins from each other. Is that accurate?
Thanks,
Rob
Just doing some back-of-the-envelope calculations…
The Wiki article on the most recent human ancestor gives the number as 2,000-4,000 years ago.
I used an average lifespan of 35 years (arbitrary, but I figure high mortality at that point would skew the average to the low end.)
So that gives you between 57 and 114 generations to that point.
Going back further, from Wiki (quoting Richard Dawkins):
Another way of looking at this is to say that our (approximately) 250,000-greats-grandparent was a creature from which all humans, chimpanzees and bonobos are directly descended. Further on in Dawkins’ imaginary journey (imaginary, in that the journey is going backwards in time), we meet the concestor we share with the gorilla, our next nearest relative, then the orangutan, and so on, until we finally meet the concestor of all living organisms, known as the last universal ancestor.
By “last common ancestor of humans”, do you mean the common ancestor of all humans alive now, or the common ancestor of all humans ever? The former is, as BlakeTyner noted, fairly recent, and so that person’s contemporaries, and parents, and grandparents, and great-great-great-great-great grandparents would have been humans, too.
You don’t want to use the average lifespan, you want to use the average age of becoming a parent. Note many people become a parent more than once so you want the average parent across all births not the average age of first becoming a parent. This is certainly lower than 35. And for most of human prehistory was probably less than 20. But using 20, the answer is 100 to 200 generations.
Good call; I’d just happened to have been curious about that question the other day, and remembered a couple of the data points from reading up on it.
My education is in the humanities, which, while it sounds like it’d make me an expert at answering this question, actually means some quick division is about all I’m good for!
Er, that is at least a couple of orders of magnitude out, dude. Even the Young Earth Creationists have humans around for 6,000+ years.
The MRCA isn’t a single individual…it changes with populations. Like the poster above said, that’s for all humans alive NOW; if you’re looking for the MRCA for all humans EVER, then it goes back much further.
Going by the estimate of modern humans emerging around 250,000 years ago, that’d be 12,500 generations (assuming 20 as a “generation.”)
To go back to the point where you get Homo Sapiens, chimps, and bonabos, per Dawkins, that’s 250,000 generations.
From the context of the rest of his question, where the OP asks also about the last common ancestor of chimps and humans, and the last common ancestor of all mammals, I think it is clear that he meant the last ancestor who was not homo sapiens, so yes, all humans ever.
Which is why I continued typing in my original response, and gave the Dawkins answer of 250,000 generations to get to the point where we see homo sapiens sapiens, chimps, and bonabos.
Just look up the time of the hominid-chimp split (5 millions years?) and divide by 20 (approximate age of reproduction).
Then you have to deal with anomalies such as the Pharaoh Ramses II, who fathered about 100 children (or so).
This is an excellent book by Richard Dawkins that addresses this question, all the way back to the dawn of life - he estimates the number of generations between various “rendezvous” points such as the common ancestor of all humans, the common ancestor between humans and chimpanzees. Excellent, if somewhat dense, book.
I recently just read a little bit about Mitochondrial Eve, and here’s the part that confuses me:
[ul]
[li]Why isn’t Mitochondrial Eve’s mother actually Mitochondrial Eve (and what about her mother, and what about her mother)?[/li][/ul]
I think based on my read of this sentence in the wiki article:
“Mitochondrial Eve is the most recent common matrilineal ancestor for all modern humans. Whenever one of the two most ancient branch lines dies out, the MRCA will move to a more recent female ancestor, always the most recent mother to have more than one daughter with living maternal line descendants alive today.” [Emphasis mine]
The implication is that Mitochondrial Eve’s mother only had one daughter, who then produced more than one daughter.
In my genealogical research, going back a thousand years, the average generation span comes out consistently around 31 years.
Note that that number varies in the last couple centuries as people started having fewer children, ending their childbearing years earlier. Also the date of first child varies in different eras. Sometimes people didn’t start families until late due to the need for the man to get his own farm or other business going. But over many millennia, the 31 year gap seems pretty good.
Thanks, BlakeTyner! When I was reading a few weeks ago I had first read a very sciencey article that one of the news sites had posted. Then I went to the Wiki page to try to understand better- but, by then I had already read too much sciencey stuff and my eyes were glazing over even at the start of the Wiki article so my reading comprehension must have been lacking.
Nitpick: Mitochondrial Eve’s mother had only one daughter with living uterine descendants.
Fathers are, on average, 7 years or so older than mothers. Since many long cited lineages are mostly male, the apparent length of generations increases.
We’ve had many many threads on this and related topics. In this post I gave the agnatic and uterine ancestries of Prince Albert (Victoria’s consort) and noted
The average agnatic generation was 33 years; uterine generation 24 years.
Completely understand! I find the subject fascinating, but there are some complexities (particularly when you start getting into any kind of math past arithmetic, lol) that I get confused about.
It looks like septimus has a better grasp of this stuff than I do, fortunately!
2-4,000 years seems too recent to account for isolated populations (such as Australian indigents, if that is the right word).
Because mtDNA Eve is the most recent matrilineal ancestor. Surely the daughter is more recent than the mother, no? 
As for the OP, pick whichever lifespan you think is most accurate and then extrapolate back. Modern Human and Neanderthal split is about 500k years ago. Human and Chimp lines split about 6M years ago.
But keep in mind that the further back in time you go, generations almost certainly become less human-like and more chimp-like, assuming our common ancestor was more chimp like in reproduction than human-like.
Even ‘isolated’ populations were probably not truly isolated. They may have, very occasionally (like once every few hundred years), had enough contact with surrounding groups to interbreed. Just imagine a lost baby or adult in the forest, adopted by the ‘isolated’ tribe (or vice versa), happening every few hundred years thousands of years ago.