Hey, how did we get here and who do we have to thank?
June 15, 2012 11:53 AM   Subscribe

How many generations am I (or you) the product of? That is, how long is the continuous sequence of paired organisms in the chain of reproduction for a human in the 21st century?

A mind-boggling thought, for those of us who might live childlessly, is that the generations before us that have successfully produced offspring go back hundreds of millions of years, with no exceptions. That is, our family trees contain unbroken sequences of reproduction all the way back to the swamp. Anyone who does not have children will be the first of their line to not do so since at least the Devonian period.

Is this correct?

If so, is it possible to estimate the number of generations that have reproduced in order to bring us here? Would any estimate be meaningful?

Furthermore, how far back is it possible to go in identifying our ancestors?

For example, is there a commonly accepted chain that looks something like this:

(single-celled organisms) > (?) > (trilobites or something?) > (?) > (fish?) > (primitive amphibians?) > (?) > (?) > (?) > (mammals) > (primates) > (us!)

Please fill in the ?s, if you can, or provide a better-informed sequence!
posted by cincinnatus c to Science & Nature (8 answers total) 4 users marked this as a favorite
I'm not going to hazard a real guess, but I will venture to say that the number will be meaningless, because the lefthand side of your equation- the single celled organisms, reproduced on a timescale of minutes, and were around for billions of years before the right hand side. So that will totally dominate the equation. I'd guess it'd be something like 10^16 generations, give or take a few orders of magnitude.
posted by thewumpusisdead at 12:01 PM on June 15, 2012

I should say that I am dimly aware of general evolutionary trees. What I am asking, in the second part of my question, is how much we can say about specific kinds of creatures we come from. For example, it seems pretty clear we came from ape-like creatures. Can we say that, before that, we came from, say, rodent-like, bird-like, fish-like, or even insect-like creatures? Or does evolution take such trips and turns that it is impossible to say which kinds of life we may have been part of or not part of?
posted by cincinnatus c at 12:02 PM on June 15, 2012

Lastly: yeah, thewumpusisdead, I guess that is true. But might there have been a point at which generations became sufficiently discrete and measurable that could say from that point something like x generations (to nearest order of magnitude)?
posted by cincinnatus c at 12:05 PM on June 15, 2012

For example, it seems pretty clear we came from ape-like creatures. Can we say that, before that, we came from, say, rodent-like, bird-like, fish-like, or even insect-like creatures?

See also: Timeline of Human Evolution
posted by vacapinta at 12:08 PM on June 15, 2012

You would love this book.
posted by theodolite at 12:10 PM on June 15, 2012 [2 favorites]

Depending on how much the question of ancestry interests you, you may want to pick up Richard Dawkins' The Ancestor's Tale from a bookstore or library...it answers the second part of your question (not the number of generations) by taking you back in time through the ancestors of our species at their important branching points. I loved the book, and it made for really intriguing reading.

Having said that, you'd have to define "first of your line" pretty tightly to see it as a big biological tragedy not to have children...our genes make it their business to be as widespread as possible, so as long as your sister, cousin, etc., has kids, your genes are happy.
posted by mittens at 12:10 PM on June 15, 2012

I too came in here to recommend The Ancestor's Tale. But note that it does include a putative number of ancestors til quite far back. It's a relative easy read, and should pretty much knock your question on the head.
posted by Jehan at 12:49 PM on June 15, 2012

OK, this is going to be very rough, cobbled together from a mixture of memory and hasty Wikipediaing, and I would appreciate corrections.

If you want to talk about our oldest ancestor that looks anything similar to us at all, you'd be looking at a fish. Specifically a lobe-finned fish similar to a coelocanth, rather than the ray-finned fish that we are most familiar with today. One of these founded the tetrapod lineage, which is everything with four limbs, digits, etc. Tiktaliik is one of the oldest fossils we have, and it's about 375 million years old (mya).

Now, phylum chordata includes everything including tunicates, which probably are somewhat similar to the first chordates. They are sort of jelly-like marine animals which happen to have a central notocord that is the most basic form of the vertebrate spinal column. They are filter feeders and exhibit bilateral symmetry as larvae (albeit with nothing so complex as a head) but, interestingly enough, radial symmetry as adults. They go back to the Cambrian period, about 550 mya.

If you want to look at the base of the kingdom Animalia, you're looking at a single-celled eukaryotic flagellate similar to the ones that you'd find today in sponges. These would be little unicellular swimmers, but still with all the basic machinery found in animal cells today -- the protein complement and cell structure would be fairly similar to what we're working with. We have fossils of these going back to maybe 670 mya.

Next you're looking for the earliest eukaryote, which is the ancestor of animals, plants, and fungi. These have nuclei, linear chromosomes, membrane-bound organelles, etc. We actually pushed the date back on these recently, and best estimates now have 2.1bya (billion years ago) as the age of the oldest known eukaryote fossil.

To go back farther you'd want to look for the LUCA, the last universal common ancestor of all life today. Everything now living traces its anccestry back to this single organism. Estimates are hazy here, but our best guess is that this guy lived maybe 3.5bya.

Finally, you have to look for the origin of life. This is different from the LUCA because, to bring up a concept both obvious and at the same time mind-boggling, there were probably many lineages that sprung off from this organism which dead-ended on other branches earlier than LUCA and which therefore did not contribute to our line. This is also a pretty hazy concept, not least because the definition of life is itself pretty hazy when it comes right down to it. Wikipedia puts the date at between 3.5 and 3.9 bya, which is probably as good as anything.

Well, there you have it -- the history of our species in reverse. That's the best I can do at the moment, given substandard conditions. (Working on Bourbon St in New Orleans, typing this on my phone mostly from memory.) I hope it goes a ways toward satisfying your curiosity and that others will come along to fill in the gaps and correct my no doubt numerous errors. Cheers!
posted by Scientist at 6:49 PM on June 15, 2012 [6 favorites]

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