Humans and Chimpanzees

Every so often you hear a TV presenter come out with something like “humans are 99% the same as chimpanzees”, or whatever the figure is. Where exactly do they pull these numbers from?

The number usually refers to the amount of DNA that’s identical between the two species. I’ve heard everything from 90 to 99 per cent, and am more inclined to believe the latter than the former.

I believe it’s a misleading statistic; I seem to recall a qualified person here on the board saying that making the same kind of comparison between humans and, say, fieldmice also results in a percentage somewhere in the nineties.

Yeas, pretty much all mammals share a huge amount of genetic material. A great deal of it is “leftovers” that are no longer used. Likewise, but to a lesser degree, with Reptiles, Birds, amphibians, etc.

Does that mean that 99% (or whatever) of human genes/cistrons are present in chimpanzees?

Also, on the same scale, how much do humans differ from each other?

Another thing that puzzles me is that huge teams of scientists have been mapping human DNA for years now, and they are still at it. How can they draw comparisons between the DNA of humans and other animals before they have finished mapping human DNA?

Humans differ very little from one another. The differences are small point mutations in big genes, many of which have no effect, but the slight changes make each person unique in the end. DNA fingerprinting works on these very small differences (Look up RFLP, this should explain more).

This is a radical oversimplification, but they’re able to do it in the same way that it’s much easier to count the number of Lego blocks of each size and color in a 500 count box and compare that with the composition of another box than it is to describe exactly how the blocks are assembled together in each box.

Take DNA that you know matches. Chop it up into small pieces, mix it together until it clumps, and see how much heat you have to apply to have it break into little pieces again. This gives you a base for comparison.

Take DNA from two sources. Chop, mix, heat. Because the parts are not all going to line up exactly right, it will take less heat to break into little pieces again.

Repeat with different samples until you have a chart of required heat. With human vs a whole heck of a lot of other organisms you should be able to show which is most similar.

I can see how this technique could be used to put animals in order of how similar they are to humans.

What I still don’t understand is where the absolute numerical value of how similar two particular animals are comes from. Is this something that can be reliably deduced from the “required heat”?

I read somewere that humans and chimps are closer together geneticly then chimps and all the other apes.

So quick lesson here…

<oversimplifying>
DNA is the stuff that all living things on earth here have in each of our cells. Each of our cells contains the entire set of our DNA. This DNA contains the information on how to create every single protien that is used to build your body, and it contains the information on how to put it all together.

Obiously, each of human is a unique individual, so each of us has differnt DNA from each other. But comparing human to human, these differences are minor compared to the vast similarities we have to one another. Something like 99.999997 (est.) is exactly the same between human to human.

As an aside, in trials where DNA is used as evidence, the DNA examinier examines a group of sequences that are known to vary from human to human.

Now then, all living creatures on earth have DNA. Even single celled organisms have it. One you compare DNA from Humans to single cell organism, you make a interesting discovery… We share about 50% (total estimate here) with them.

How can that be? Hey, making a cell is pretty complicated stuff… there are 20-35 organelles in each cell, not to mention cellular membranes and cell wall for plants, all of which needs to be described by DNA. We certainly have a lot more cells, but each or our cells is constructed with similar organelles. Thus, our DNA needs to contain the information on how to contruct cells as well, so it’s not completely suprising that we have some in common with single cell organisms.

When you look at complex creatures, such as a fish, they have a lot more DNA. We share maybe 60% (total estimate here) with a fish.

Again how can that be? Well, making fish is pretty complicated stuff, too. In addition to the whole making cells thing, they have the DNA nessasary to orgainize those cells into complex systems (eyes, hearts, gills, blood, muscle, even a bit of brain). And hey, we need most of those too. Thus, we also contain the DNA to make many of those structures.

If we extrapolate this out, we see that any creature with lungs (a bird) is closer to us than a fish (say 70% DNA similarities) mammal such as a dog have even more DNA in common with us (say for the sake or argument at least 80%)

Finally, we look to the creature that look very similar to us… Monkeys and Apes. According to this website http://www.gate.net/~rwms/primegendist.html, a chimp has 98.4% DNA in common with Humans.

Should we find that suprising? Not really. Even if you knew nothing of our planet or of the theory of evolution, and were asked to pick out the species of creature on this planet that is closest to humans using only your eyes, chances are you’d pick a primate over a fish any day. They sook similar to us, have all the same internal organs, move similar to us (at least more so than a fish), they even act similar to us in some ways. Of course, they don’t use personal computers, but then again, they are lacking that 1.6% of DNA that makes us human.

As another aside, while I did estimate the percentages in the earlier examples, the ratios (more DNA in common with mammals than fish, more in common with fish than single cell orgainism) is essentially correct. I’ll leave it to somone else to find the actually values if they are nessasary) But, the Human vs. primates data is considered fairly accurate. All of this information on genetic similarities is a very good arguement for evolution.
</oversimplifying>

One way that an actual number was determined was by looking at specific proteins. Since proteins are made by the cell based on the information coded into DNA, then the are a representation of the DNA itself (Imagine a picture of something…it isn’t the thing itself but it pretty accurately descibes what the thing is). Proteins are pretty big molecules, so they are relatively easy to isolate, and many MANy proteins have been characterised and have had their sequences (and therefore the gene sequence) determined. By comparing a specific protein in humans to the same protein in another animal, such as a chimpanzee, you can see how much it varies across species, and therefore how much the DNA might vary.

One example is that of the cytochrome c gene, which is well studied. Although not an official source, this page http://www.millersv.edu/~bio375/EVOLUTION/NEUTRAL/NEUTRAL.htm shows a quick table showing how many amino acids are different in the cytochrome c protein across species. The Rhesus monkey only differs by one amino acid (one DNA codon) while yeast has 42 differences. This is a measure of genetic similarity. By doing the same comparison for many more proteins, it is possible to give a better estimate of how similar we are to other species. This also explains the wide range that people find reported (90% the same or 99& to chimpanzees), because it all depends on what you were studying. Hemoglobin might be very similar while something else (keratin, I think, in hair) might be quite different.