Okay, time to ramble on a bit about that other branch in the conversation... Rich started it off by asking:
Suppose we have a big computer with everyone's genome data. What can we deduce about the genomes of our ancestors? Could we reconstruct an estimated genome of Fermat, Buddha, or Caesar?
The more straightforward question is what we can deduce about the family tree of all people currently alive. Tom Knight already mentioned the deCode project to create a genome map of the entire population of Iceland, which is remarkable for both its scientific value and its tricky ethics. If we were haploid -- that is, we reproduced asexually, and you were just a slightly imperfect clone of one parent -- then this would be a well-studied problem. That kind of thing is done all the time at the level of species, not individuals; it's called "phylogenic tree reconstruction". It's an area of active research, and people are getting pretty good at it; that phrase will allow interested folks armed with Google to learn more. In that case, the methods do indeed produce a family tree in which each dedeuced ancestor is labelled with a most-likely genome -- though the problem of determining which node on the tree corresponds to Fermat/Buddah/Caesar remains a sticking point. But our knowledge of the mutation rate in human DNA is good enough that we can pretty well tell how long ago the various individuals in the tree lived -- so applying this to the Y chromosome, we can indeed say that *someone* around the 1200s was a direct male-line ancestor of about 8% of all men alive today in the areas the Mongols visited, and by looking at historical records we can make guesses about it being GK himself, and can make a darn good guess as to what his Y-chromosome was. (GK is probably *ancestor* of everyone there; for 8% of the people, he is their father(father(father(...))), which is a *much* stronger statement.) In the same way, we can tell that some male less than 100,000 years ago is the direct male-line ancestor of all males alive today. But the hopes of reconstructing the full genomes of long-dead people are completely dashed by that messy sex stuff. Mike Stay has already plugged Dawkins's book "The Ancestor's Tale", and I'd like to loudly second that recommendation; it's a superbly-written introduction to how to think about all this. (Note to self: next time someone asks me for a pointer to a book to learn the math that goes with all of this, push this instead!) At the risk of repeating some of what Mike wrote yesterday... For any population -- say, all Europeans, or all humans alive today -- you can ask, who is the Most Recent individual who is a Common Ancestor to all of them? This MRCA changes over time. Before the 1400s, eg, the MRCA(Europe+America) was surely from from before the last time there was a Bering land bridge, maybe 20,000 years ago. But today there are probably no pure-blooded descendants of the Paleo Indians left, due to either lineages dying out or intermarrying, and that MRCA is surely more recent than that. (Estiimates of MRCA of all humans vary wildly, and I don't know enough to get into that debate.) More surprising is that, once you go back far enough that *someone* is an ancestor of, say, all people living today, you don't need to go much farther back before *everyone* is either (a) an ancestor of everyone alive today, or (b) an ancestor of no one, a person whose line has died out. Moreover, about 80% of individuals will be (a)'s. Of course, barriers to communication between separated groups of humans get in the way of this happening, but such barriers are almost never absolute. Note that this pretty well puts the kibbosh on reconstructing Caesar's genome: his descendants might include, say, everyone in Europe, but so what?, that doens't distinguish him from 80% of the population of Italy at that time. Let me just add that when you first hear this, you may well have the overpowering reaction "What, what about natural selection?!" After all, if the fittest have the most kids, but at the same time 80% of people end up ancestors of everyone, where's the advantage to being fit? Dawkins addresses this superbly: natural selection doesn't happen on the level of individuals, but on the level of genes. Every *gene* has only one parent, and we're back in the nice clean world of mostly-perfect clones.
If we throw in present-day location information, we can probably figure our who moved where, when, and perhaps deduce population movements from long ago.
The human HapMap project, a large amount of which happens in the building where I'm typing this, can do this and vastly more. But I'm out of time, so that will have to wait for another day. --Michael Kleber -- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.