October 13, 2006
Looking for deep ancestors
Richard Dawkins in his book The Ancestor's Tale (2004) tells a fascinating story. He models his book on a journey that, rather than moving through space to a particular destination, is moving in the temporal dimension, going steadily back in time. He calls it a "pilgrimage to the dawn of evolution." He starts with present day humans and follows them back into history. One reason he gives for going back in time instead of starting at the beginning and going forwards as is more commonly done is to avoid a common trap of perception. When you tell the story forwards, it is hard to avoid giving the impression that life evolved purposefully, that human beings were somehow destined to be. This is counter to evolutionary theory that says that evolution is not directed towards any goal. It tells us how the present emerged from the past. It does not tell us how the future will emerge from the present.
Dawkins points out that the another advantage of telling the story backwards is that you can choose any of the current species and go back in time and tell pretty much the same story.
As I have mentioned earlier, we quickly (in just 2,000 years) reach the time when the most recent common ancestor lived and soon after that (about 5,000 years ago) reach a point when all our ancestors were identical.
But this convergence of ancestry is not just for humans, it is for all species. If we go far enough back in time, even my dog Baxter and I share the same ancestor, which I find a very appealing notion.
Anyway, here is a concise summary of the landmarks on this pilgrimage back in time, along with some other landamrks.
About 10,000 years ago, the agricultural revolution began and about 12,000 years ago saw the beginnings of language. About 160,000 years ago saw the beginning of what we would consider modern humans, and beyond that we start reaching the precursors to modern humans, a famous milestone being the fossil Lucy, dated to 3.2 Mya (million years ago).
As we go further back in time in this pilgrimage, other species start 'joining us' in our journey. What this means is that we reach times at which an earlier species existed which then split into two branches and diverged evolutionarily to what we see now. So if we go back further in time, we should cease to view the pilgrims on the journey as a combined group of humans and other species but instead see the travelers as that earlier common ancestor species. He calls these common ancestors 'concestors'. (Concestor 0 in Dawkins' scheme is the most recent common ancestor of all humans (or MRCA) that I have discussed earlier and who lived just a few thousand years ago.)
Going back in time, at 6 Mya we meet concestor 1 when we join up with the ancestors of chimpanzees. As we go even back further, we (and when I say 'we', I remind you that we should not think of 'us' as humans at this point but as the common ancestor species of humans and chimpanzees) join up at about 40 Mya successively with gorillas, orang utans, gibbons, and finally monkeys. Remember that the 'pilgrims' look different as we pass each concestor point.
Concestor 8 occurs at about 63 Mya when we join up with mammals like lemurs and lorises. (Just prior to this, around 65 Mya, was when all the dinosaurs went extinct.) As you can imagine, concestor 8 would not look much like present-day humans at all.
About 75 Mya, we join up with rats, rabbits and other rodents (concestor 10), at 85 Mya with cats and dogs (concestor 11), at 105 Mya with elephants and manatees (concestor 13), at 310 Mya with snakes and chickens (concestor 16).
At 340 Mya, we make a big transition when join up with the ancestors of amphibians, such as frogs and salamanders (concestor 17). This point marks the first time that animals moved out of the water.
Around 440 Mya we join up with various kinds of fish (concestor 20), and around 630 Mya with flatworms (concestor 27).
After various other species ancestors' join ours, the next big rendezvous occurs at about 1,100 Mya when we join up with the ancestors of fungi, such bread molds and truffles (concestor 34).
Some time earlier than that (passing the connection with amoeba at concestor 35) but before 1,300 Mya (it is hard to pin the date) is when the next major transition occurs when we join up with green plants and algae. This common ancestor is concestor 36.
At about 2,000 Mya we arrive at concestor 38 where every species is now represented by a eukaryotic (nucleated) cell.
At about 3,500 Mya we meet up with our earliest ancestors, the eubacteria (concestor 39), the original form of life.
Dawkins' reverse story can be seen visually, told in a beer commercial in 50 seconds flat to the pounding beat of Sammy Davis Jr. singing The Rhythm of Life. (A minor quibble: There is one way in which this fun visual representation is not accurate. It shows three humans going back in evolution until we join up with ancestors of the present-day amphibians (concestor 17) in identical parallel paths. This is ruled out by the reductio ad absurdum argument written about earlier, where it was established that all present day humans must have had a single common ancestor in any earlier species.)
I must say that this book was an exhilarating journey. To see the whole of the evolution of life going backwards and merging together was a nice new way of seeing the process. Those of you who are interested in the grand sweep of evolution written for a non-specialist will find Dawkins' book a great resource.
POST SCRIPT: The Boxer
A live performance of Simon and Garfunkel singing one of my all-time favorite songs The Boxer