Suppose I put a picture of myself on page 1 in a photo album.  Then a picture of my mother on page 2.  My grandmother on page 3.   Then, using a time machine, *her* mother on page 4, and her mother in turn on page 5.  And I keep doing this.  Referring to … _evolution I'll have a picture of a picture of a distant ancestor who was a great ape from around 13 million years ago (MYA).  Going back further still, a picture of something like a mouse from 100 MYA.  A reptile from 300 MYA.   A flatworm around 550 MYA.  And let's stop there, my earliest ancestor to have a brain. 

How many generations will I have found between a flatworm, and me?   

Just trying to come up with an order of magnitude estimate, primate generations aren't all that short, but primates are a small percentage of the timeline and things speed up as you go back. Googling for "generations per year" and looking at various sorts of insects and rodents and other creatures, 2 to 10 generations per year seem to be numbers that show up a lot.  So maybe an average of five generations per year overall?   So, maybe a few billion generations between a flatworm, and me?  Call it an average of ten generations per year and we're still only up to six billion or so. 

Is that the right ballpark?   Doesn't it seem like too small a number?  But if so, where am I going wrong?

Hi Ulia,

Sounds like you will need a pretty big photo album!

As you are obviously aware these kinds of calculations can only ever be estimates, but they are interesting to think about - I think Richard Dawkins did something similar in "The Ancestors Tale".

Personally, I think your six billion figure may be an overestimate. If we follow our ancestral line back to 300 million years ago we will find a fairly complex vertebrate called a lobe-finned fish (these are still alive in the form of lungfish and the coelacanth). I can't see these animals reaching sexual maturity in under a year. So even if we assume that average generation time over this period was two years (probably an underestimate) then it would only be 150 million generations.

You are probably right that generation times would get shorter as we followed our own ancestral line backwards in time. But for other organisms like bacteria their generation time has probably not changed measurably in hundreds of millions of years.

So, I have a question for you to think about: what effect does generation time have on your ability to evolve?