In many ways our DNA defines us. Consider identical twins, which are the result of an embryo splitting in early life. They are actually a single individual that divided into two parts during the early stages of embryonic development. They don’t call them identical for nothing. Studies show that in addition to looking the same they are also extremely similar in almost all other measurable respects, including intelligence. And, of course, the reason that they are so alike is that they have identical DNA.
But what makes most people so different from each other? We are only beginning to understand how our DNA differences distinguish us. The Human Genome Project, finished about a decade ago, was a huge first step. It defined the sequence of the A, T, G and C building blocks of our three billion bases of DNA. Like the Moon Project, it was an amazing accomplishment for mankind.
In a sense it was like our DNA defined itself, through us. Our DNA, which encodes us, was actually able to determine its own sequence, using people as a tool for the process. A very smart molecule indeed.
One of the most incredible discoveries to come from the Human Genome Project was the remarkably small number of genes we have. It turns out that we only have about 25,000 genes!! This seems a pitifully puny number.
Think of it. We all start a single cell, a fertilized egg, a very small speck of protoplasm barely visible to the naked eye. And this one cell somehow turns itself into a complete person. And people are incredibly complicated. They can weigh one or two hundred pounds or more. They have arms and legs and complex organs like the liver, heart and kidney. And they have a brain, which has about 100 billion neurons with over 100 trillion specific interneuronal connections. Wow!! All of this from just a single cell equipped with a genetic program of only 25,000 genes in its nucleus. Truly amazing.
And then we sequenced the DNAs of several people, and compared them. It turns out that our sequences are about 99.9% identical, from one person to the next. Our individual differences are defined by the 0.1% sequence variation, the several million bases that fluctuate among the three billion total. As we sequence the DNAs of more and more people we will begin to define the complex equations that relate these sequence differences to specific traits.
And when we sequenced the DNAs of other animals we made more interesting discoveries. It turns out that all mammals-including lions, cows, pigs and elephants- also have three billion total bases of DNA, and about 25,000 genes. Indeed, we all have pretty much the same set of genes. To paraphrase Mario Capecchi, a Nobel laureate, who I once published a paper in Nature with, mouse and man are 99% genetically identical. What he meant by this is that for every gene a person has there is about a 99% chance that the mouse will have a corresponding very similar gene. All mammals have the same genes, but those genes will differ in exact base sequence from each other. His point was that the mouse is an excellent genetic model system for scientific study, because it so closely recapitulates man.
But another key conclusion is obvious. All mammals are very closely related.
Of course the chimpanzee is the extreme example of this, with DNA sequence that is about 99% identical to that of man http://www.seattlepi.com/local/article/Chimp-human-DNA-comparison-finds-vast-1181942.php. The chimp doesn’t just have the same set of genes as man, indeed about one third of chimp genes encode proteins that are exactly the same as their human counterparts http://en.wikipedia.org/wiki/Chimpanzee_genome_project. This sort of result is exactly what evolution predicts, and creationist/anti-evolutionists have to struggle very hard to explain it http://www.christiananswers.net/q-aig/aig-c018.html
But instead of looking at close relatives, let’s look at very distant relatives, like the fruit fly. Shockingly there is some fascinating evidence that we are even genetically related to these little bugs.
Geneticists discovered some crazy mutant fruit flies about a century ago. One mutation, for example, gave a fly that had legs growing out of the head where the antennae were supposed to be. Imagine that! During development the cells that were supposed to make antennae instead made legs that now protruded from the head! Another interesting mutation gave a fly with no eyes.
As technology advanced it became possible for us to manipulate the genes of the fly. We could alter the gene involved in leg/antennae development and re-create a fly with legs coming out of the head. But even more remarkable, we learned how to introduce human genes into the fly. And when we made similar alterations to the human counterpart of the fly gene, and put it in the fly, we got the same result, a fly with legs on the head! The human gene seemed to be functionally equivalent when placed in the context of the fly. A truly surprising and amazing discovery!
We could also take the fly gene found to be critically important for making eyes and cause it to be mis-expressed during development. It was found that if this gene was activated on the legs, for example, one could make a fly with extra eyes on its legs! Indeed it was possible to make a fly with lots of eyes, on the legs, on the antennae, and at other places on the body. But, once again, the most remarkable result came when we took the human gene most closely related to the fly eye gene and placed it in the fly. Just like for the fly eye gene, we found that if we turned on the human gene at various locations during development we could make a fly that had fly eyes all over its body. Once again, the fly and human eye genes appeared to be functionally equivalent.
These results show a remarkable, and quite unexpected, conservation of developmental genetic programs stretching across enormous evolutionary distances. Many human genes seem to work quite well, thank you, when functionally tested in fruit flies, where they initiate genetic cascades that drive the formation of discrete fly body parts. That is, in many cases fly and human genes are functionally interchangeable.
These experiments demonstrate evolutionary relatedness not only between chimps and people, but also between insects and people.
And this is one way that DNA proves evolution.
About the Author: Steve Potter, PhD, is a Professor of Pediatrics, in the Division of Developmental Biology, at Children’s Hospital Medical Center in Cincinnati. He has authored Designer Genes: A New Era in the Evolution of Man, published by Random House 2010 http://www.amazon.com/Designer-Genes-New-Era-Evolution/dp/140006905X/ref=sr_1_1?s=books&ie=UTF8&qid=1310842010&sr=1-1. In addition he has written over one hundred science papers, and co-authored the third edition of the medical school textbook, Larsen’s Human Embryology.