Monday, December 02, 2013

What makes humans different from other sorts of organisms?


I've heard a lot, in Genes and the Human Condition, about what makes us the same as others, and what makes us different.

My very favorite graphic, so far, shows a section of DNA as it is found in chickens, in chimps, and in us.  Mostly the same in all of us, but................

I didn't find a copyright-free version of the image to show you.  If you click on this link, and scroll down, you will find a mostly-gray rectangular image in the middle (left/right-wise) that has pics of a man, a chimp, and a chicken.  In a magenta band at the top of the image, it says "Scanning the Genome."  The image shows the sequence of DNA, as it's found in chickens, chimps, and humans.

You may recall my reporting from Dino 101 that there is clear and compelling evidence that chickens (and the rest of the birds) are descended directly from dinosaurs.  You may also recall my post about dimetrodon, which talks about how we know we are more closely related to dimetrodon than we and dimetrodon were related to dinosaurs.

The last common ancestor for chickens (and dinos!) and chimps (and us) was before dimetrodon.  Before synapsids and diapsids went their separate ways.  Long before the first dinos walked the Earth.

In Genes, the estimate for the last common ancestor for chickens and chimps (and us) was 300,000,000 years ago. Chimps and chickens show almost the same DNA sequence, even after 300,000,000 years of separate evolution.  (Which is kind of amazing in itself, don't you think?)  Dinos, from whom chickens have inherited this DNA, presumably had the same sequence we see in chickens today. 

(Digression -- If I understand correctly what Ive been taught -- when a section of DNA is as similar as this between two organisms that haven't had a common ancestor in 300,000,000 years, we believe the DNA is unchanged over that amount of time.  This indicates DNA that is essential to an organism -- most changes to this DNA are fatal.  End of Digression.)

Katharine Pollard and her team look for the areas of the human genome that differ the most from the chimp genome.  H(uman) A(ccelerated) R(egion) is the name Pollard et al have given to sections of DNA that are the most different between us and chimps.  The HARs are numbered by the degree of difference.

The section of DNA in the graphic is HAR1.  It is the section of our genome that shows the most difference from the corresponding section in the chimp genome.  HAR1 is about brain development.

Notice that the difference in this sequence, between chimps and chickens after 300,000,000 years of separate evolution is only two changes.

Current thinking is that we and chimps had our last common ancestor 7-7.5 million years ago.  In that relatively short span of time, our version of the HAR1 section of DNA has changed by 18 differences from the chimp version of this section of DNA.

(Digression -- Don't you think this was a smart way to look for the things that make us the most different from chimps?  (Note also the illustrations that follow the one I'm talking about -- HAR2 is about development of wrists and thumbs........  End of digression.)

My very favorite thing about the graphic is that they chose a descendant of dinos to include.  (For all I know, they could have included a turtle.  Or a fruit fly.....  But they picked a dino's child.............)

I am tickled to have that chicken (and her dino ancestry!) included in the info about genetic change over time.

I love it when what I learn in one place connects to and enriches what I learned somewhere else.........


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