Wednesday, November 12, 2008

book review

Carl Sagan and Ann Druyan are the authors of this bestseller, Shadows of Forgotten Ancestors: A Search for Who We Are. I see it rates 4.5 out of 5 by reviewers on, where you can pick up a used copy for something like 30 cents.

I found two specific portions of this book most interesting. The first, starting around chapter 5, details the code sequence of DNA. It is made up of only 4 unique molecules, called C, A, G, and T (the first letter of the scientific names: cytosine, adenine, guanine, and thymine). If you think that only four "letters" or "molecules" could hold enough information to create the richness of human life, consider that computers only use two. The pictures on this blog, the movies you watch, the calculations computers do all relate back to only Y or N ... yes, or no ... on, or off ... 1 or 0 is how it is usually described. The music of Bach on an audio CD is the same, only 2 letters.

So, the DNA is made up of a string of these molecules, and we represent them by a sequence of letters: CAGGTTACCTATGACCTAG ... and in humans they go on for about five feet (much of it never used, for what it is worth ... good thing, too, since maybe the unused portion says to grow gills or scales or something). Not 5 feet of letters on a page or screen. Five feet of molecules, all attached, in a few long, fragile strings called chromosomes. Usually they're tangled up like a ball of yarn, and part of how they work is by untangling a little bit of the string that has the gene sequence that needs to complete some task, like creating a digestive enzyme or something. This sequence, and this was news to me, makes up 64 unique 3-letter words. Like CAG or GTT or ACC using the first few from the imaginary sequence I used.

That makes me feel better. I'm a very complex machine, and the idea that everything I am, and do, and think, and feel are controlled by 4 basic information units was a bit unnerving. Now 64 discreet states, that's more like it. As an aside, a new electronic component now joins the resistor, capacitor, and inductor. It is capable of memory of a previous voltage level after the power is removed, and is called a memristor. It can remember more than "on or off" and will revolutionize computing. You laugh now, but wait until some uppity 128-state processor disses our biological weakness.

Scientists have now documented the human genome, the sequence of DNA molecules that make us who we are. Of course, much is similar between you and I. There are differences, too. My eyes are hazel. Maybe yours are blue. Our genetic code is different.

But what is amazing it how much of it is similar! Not between you and I, but between me and a chimpanzee. The most similar genetic code to human DNA is that of chimpanzees. The book notes, "When ACGT sequences that are mainly active genes are examined, a 99.6% identity is found between human and chimp." Did I mention that a lot of the strands of DNA contain unused information? "In the kangaroo rat of the American Southwest, for example, the sequence AAG is repeated 2.4 billion times, one after the other ... ."

Perhaps one day radiation will cause a random mutation and this kangaroo rat tabula rasa seems an obvious place for nature to experiment.

It is worth noting that the common factor in life, as we know it, is DNA. Even unusual things like viruses have genetic codes, though theirs is more geared toward taking over an existing cell and putting it to work to make copies of the virus. Even those strange life-forms that live in the darkness of the deep ocean, clustered around a thermal vent, have genetic sequences.

The second portion of the book that I found most interesting was one on which the authors spent a lot of time and wrote many chapters. Chimpanzee behaviour and communication. One thing that surprised me is that chimpanzees can communicate about a plan for a future action regarding a hidden object, and we humans still have no idea how they do it. Chimps use deception, make and use tools, and practice warfare. Sounds like the history of civilization to me! This is a fascinating book, and it gives insight about what it means to be human. I highly recommend it.

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