Re: 93-95% Gene Similarity between us and apes...does it mean ANYTHING?

Adam:

This is a really good question, and with a large amount of discussion in the media today about genome projects, and evolution in general, this is an apt time to discuss this. First of all, we need to clarify what 'We' mean when we say something is 95-99% similiar to us, such as a Chimpanzee, which is estimated to be as much as 99% similiar to humans at the DNA level.

You are absolutely correct when you say that small genetic differences have huge effects: one nucleotide (what you call a nitrogen base), or one amino acid can have vast implications in the function and structure of a protein. For example, the genetic difference between normal and sickle-cell red blood cells is in fact, a single change in a DNA nucleotide, which causes a single amino acid to change.

You have a disconnect in your logic here. Just because two things are divergent, and have distinct differences at either the DNA or the protein level, does not mean they are unrelated. You would certainly agree that someone with sickle-cell is related to their parents, although neither parent has sickle-cell anemia, wouldn't you? The presence of differences does not rule out relation, essentially.

As scientists, we use relations and evolution to help us organize and explain the data we observe. By examining relations between organisms, we can start to predict what the function and structure of compounds and proteins might be without additional experimentation, just because we have seen something with a similar sequence before. For example, studying simple organisms such as yeast, which are easy to grow, and manipulate genetically, is valuable for human research, which is difficult to do for obvious reasons. This is because many of the proteins in yeasts and humans, are homologs of each other, that is they are related through evolution. We know this for a variety of reasons, but the main one is similarity of sequence. If you sequence two genes from two different organisms and compare them, you will see differences in certain places. The level of differences between them may be great or small, but the more identical things are, the greater the odds of their having a common ancestor. What is really cool, is that if we delete a particular gene in yeast (Baker's yeast) and replace it with the human gene we think is homologous, we can 'rescue' the yeast and restore the activity it lost.

In many cases, proteins can be related, but have completely different functions, this is how the vast array of proteins we have can evolve from just a few common proteins. Nature doesn't have to be perfect, it just has to work, so it is continually not re-inventing the wheel. If a structure has a certain activity that is chemically similar to another that would be advantegeous, it is much easier for evolutionary pressures to modify what is currently in the organism to gain an activity than it would be to evolve the new function from scratch.

The truth is, that the differences between these organisms are much more interesting that the similarities in many ways, and they actually do as much to support evolution and relations as the similarities do. As for your exclamation of using pig organs because they are similiar, this is a completely different issue, and we mostly use swine organs for a couple of reasons, none of which have to do with evolution:

1. Pigs at a young age are about the same size as humans, so the organs are appropiately sized.
2. The husbandry of breeding and raising pigs is well known, and a large supply exists. Breeding chimpanzees, by comparison is very difficult.
3. Latent moral objections to using primates are not generally present in pigs for obvious reasons.

Despite this, pigs are actually not a suitable organ donor at this point. We have great difficulty in maintaining organs between closely matched human donors (immunologic) and utilizing pig organs is an even greater challenge. Several xenotransplantations with baboons have occured over the past decade or so and none of them were successful for more than a short time due to organ rejection.

This is getting long, but you have addressed a very important issue in society today, and it is worth an extended explanation. Eugenie Scott, head of the National Center For Science Education recently wrote an essay in 'Science' on evolution, I have included the URL for that essay below. She includes several evolution references from the popular press that you can examine if you wish.

Thanks:

-Matt-

http:// www.sciencemag.org/cgi/content/full/288/5467/813