[Note this was written to follow on from discussions which began in comments on post "When you wish" below and continued into those of "Extraordinary". One of my most regular commenters on this blog, Eric, is trying to understand evolution. I, we, are trying to help him. This post arose from Eric's comment that "I don’t get the 'every generation' being a transition between the species at all." So, Eric, let's try it like this.*]

I don’t know if you are interested in your family history, but let’s assume you are. And let’s assume that you know all your ancestors, way way back (will come to how way back soon). And let’s imagine that not only do you have photographs of your ancestors going back 150 years (when the camera was actually invented) but there was a previously unknown mechanism which enabled photos of all your ancestors going way way back.

So you start to arrange the photos on your table. Parents first, then grandparents, great grandparents and so on, back through the generations. And let’s assume (last assumption, promise) that you have not only told your immediate family what you are doing, but have told your more distant family of cousins etc, your community, and, through the internet, the whole world.

OK, with me? Right. You are putting your great grandparent’s photos on the table, your children remarking how much you look like them and you not being sure if it is a compliment, when in walks your second cousin and says, hey, they are my great grandparents. You chat for a while and keep working. Down go the photos generation after generation.

Back to six generations and a previously unknown cousin from Germany drops in and points out you share a great great great great great grandparent. He still lives in the same village your ancestors migrated to America from (I have a real example of that, a sixth cousin living just a few miles from the village in England my six times removed grandparents lived in).

I don’t really know your background so I will switch to me now. I keep adding more and more generations (roughly 4 per hundred years). Back a thousand years. All those people, all 45 generations look much like me – variation in hair colour here, different height there, shorter nose over this way – all residents of the English Midlands.

About a thousand years ago a bit of a change – more men and women seem to be of strapping solid build with red or yellow hair. Just as I am putting their photos out, trying to keep track of which generations I am up to, there is a knock at the door and a couple of strapping red haired gentlemen tell me that I have just identified some common ancestors of their’s which means I now have Danish and Saxon distant cousins.

Another 500 years back and a knock at the door tells me those dark haired olive skin ancestors are the reason I have Italian cousins. And so on, back thousands of years. Little differences between generations, but all recognisably the same group. Tens of thousands of years, still the same, and if you put any of them in modern clothes they could drink in my local bar unnoticed.

Oh there are changes, as you [that is me, "I" became awkward!] realise when you look back to your recent ancestors, and when you hear knocks on the door from people from Germany, Hungary, Turkey. But still, generation to generation, no obvious change. And then as more tens of thousands of years tick over you get vists from Australian Aborigines, Asians, South Americans, and finally Africans. Still no obvious change from generation to generation, but your ancestors are now recognisably different – shorter in stature, with curly hair and darker skin – from those ten or twenty thousand years later or those today. You realise if you put the photos in a bundle and flicked them like those old children’s moving picture books, you would see a gradual change over time to the present day.

And still you go back, ancestor after ancestor. You’ve lost all sense of time. What is it, a million years maybe, gosh, that’s, um, 2500 generations. Curious, you hadn’t noticed change, but these G G G G …. Grandfathers of yours are much heavier built, more muscly, bigger jaws, bigger brow ridges. Other cousins drop by from Indonesia, far east Russia. Similar but a bit different again.

And still we go on, another 2500 generations and another. Is it your imagination or do these ancestors seem shorter, darker, more, well, hairy? No it’s not. You look along the table, can’t see the change, until you jump forward a couple of million years (big table this) and compare. And then you get a phone call from zoo, chimpanzee there wants to tell you that you and he are 10,000th cousins, sharing a 9999 great grandparent. Seems odd, he looks quite different to you when you arrive at the zoo, but on a table he has arranged all his ancesors going back same number of generations as yours. You look at his photos and notice the same pattern. The later ones all look just like him, “typical chimpanzees”, but as you get further back you notice small changes – slightly shorter arms perhaps, jaws a little larger, hair colour a little different, slightly more upright. He points at another slightly different looking chimpanzee, and you discover that when your friend got back to about a million years ago he got a visit from a Bonobo chimp who said he was his long lost cousin, just like the calls you had along the way. By the time you look at his ancestors and yours from about, say, 9000 generations ago, there’s not much difference at all, and when you get back to the shared ancestor they are of course identical.

Seeing that you are a bit puzzled your Chimpanzee cousin points at you and points at the ground, then points at himself and then at a tree outside. Light dawns – your common 9999 great grandparents were mainly ground dwellers, but around 4 million years ago his direct ancestors were in a group that became separated from yours, and while your ancestors adapted more and more strongly to ground living, his were in an area where tree living was all the go. Adaptation proceeded in the two directions in different parts of Africa, and by the time conditions changed and the two groups were in contact again they had become different enough not to interbreed.

You go home, pleased to have discovered more long lost cousins, and keep working through your photos. Back to 7 million years, 17500 generations and another zoo call, this time the gorilla wants to say hullo to his cousin. Same thing. A line of photos on a table, call from a distant gorilla cousin (separated in east Africa), not much change from one photo to next, but change over longer time. Seems quite different to Humans and Chimps initially, but doesn’t look that much different to the common human-chimp grandparent perhaps 5 million years ago, and as you get further and further back they converge in appearance (and genetics of course) until they look more and more like a kind of generalised ape – Australopithecus (again with various cousins).

And… Well, you get the idea. You can continue generation after generation through the other apes, then back through the early mammals, the reptiles, the amphibians and so on. Not much evident change from one generation to next but over immense time substantial change. No modern species the direct ancestor of any other modern species, just like your cousins are not your grandparents, but all are cousins to some degree.

There you are Eric, evolution over four billion years in a short post – gradual adaptive change, and equally importantly, geographic separation of different populations forming new species. What a wonderful world that has such potential in it.

* I’d love to claim the credit for this idea of how to present evolutionary change, but saw it (in the marvelous illustrations by Dave McKean) and read it originally in Richard Dawkins 2011 “The Magic of Reality” Random House, London. However I have added the calls from cousins, and the zoo, as my own piece of originality.