Ask a Geneticist

by Monica Rodriguez, Stanford University

If evolutionary theories hold true, how did the leap from 24 to 23 chromosome pairs occur from 'apes' to humans? How is it possible for two chromosomes to merge into one, and for the resulting species to survive to breed?

-A high school student from the UK

August 29, 2007

Human chromosome 2 is made up of
two ape chromosomes.
Some very tough questions you've just asked! I will first start out by saying that scientists don't know for sure why our ancestors split into two different species.

But we do know that chromosomes can stick together. And we know that people who have two chromosomes stuck together usually have no problems.

How do we know this? Because there are lots of people like this running around. Something like 1 in 1000 live births has this kind of chromosome mix up.

So there isn't really an issue with the chromosomes sticking together. The tricky part of your question is how this changed chromosome could have taken over the population and became what is most common in people.

We'll go over some of the ways this might have happened later on. It is important to say up front that we don't yet know exactly what happened. And that we may never know for sure.

But before going over this, let's talk a bit about how we know that one of our chromosomes is actually made of two ape ones. And why chromosomal rearrangements don't cause more of a problem.

You are right that evolutionary theory says that humans evolved from an ape ancestor who had a different number of chromosomes than modern day humans. Humans have 23 pairs and apes have 24.

The theory is that at some point two ape chromosomes fused to make a single human one. Why do we think this? Because when we look at human DNA, chromosome 2 looks just like two ape chromosomes stuck together.

All chromosomes have a distinct structure at the ends (called telomeres) and at the center (called centromeres). By looking at the DNA we see that our large chromosome has evidence of a telomere in the center. And we also see evidence of two centromeres. So this means that most likely two chromosomes fused their ends together.

Like I said, this sort of thing happens all of the time. Chromosomes can break apart and they can fuse together. A part of a chromosome can flip around or a part can move to an entirely different chromosome. We see examples of all of these things when we look at human and ape (and any other) chromosomes.

You might imagine that changing something so important as a chromosome would be a disaster. Some creationists actually use this point as an argument against human evolution. But we know that chromosome rearrangements aren't always harmful.

This is because chromosomes are really just long strings of information. These strings are in discrete chunks so there are lots of places where they can be broken and pasted back together again.

As long as the information is all there, it mostly doesn't matter how it is packaged. The information that the ape has on the two 'split' chromosomes is the same as in our large chromosome 2. So even though we have one less chromosome, all of the information is still there.

As I said before, this sort of chromosomal rearrangement actually happens in about 1 in every 1000 babies. It's called a Robertsonian translocation. A part of one chromosome joins together with another chromosome and just like with the ape chromosomes, no information is usually lost. So this person is completely normal despite having the translocation and one less chromosome.

But this can be a problem when the person tries to have a baby. Half the time their children are fine. These kids are either normal or carry the translocation.

Half of the time, though, a fertilized egg will inherit a missing or extra part of a chromosome. Most commonly, this results in a miscarriage.

How can we get to 23 pairs in the general population?

About 1 in 1000 people have major
rearrangements in their chromosomes.
So it is easy to see where the first translocation came from. These things are pretty common after all. But how could it spread? Especially if these folks are at a disadvantage because they have more miscarriages.

One way we may have all ended up with 23 pairs of chromosomes is if there was some advantage to having the chromosomes fused together. This would counter the disadvantage of increased risk of miscarriage.

Then we can easily see how it could spread in a population. This is how natural selection works after all. But we have no evidence to support this.

Another way is through chance. Chance is how DNA changes can spread in a population even when there is no advantage.

This sounds weird but is actually common enough to have scientific names like founder effect and genetic drift. The basic idea is that in small populations, random effects can have profound consequences.

If you start out with a small population, then there is less DNA diversity in that population. This is why, for example, certain diseases are so common in Amish communities. And others are unheard of.

When these populations were started, the founding members either had the disease genes or not. If they had them, then the disease became common. If they didn't then the disease did not exist for them. This is the founder effect.

To make this as simple as possible, imagine a situation where two of our ancestors had a fused chromosome. These two leave the tribe and found a new group.

They have six kids. Two of the kids inherit a fused chromosome from each parent. They now have one less pair of chromosomes.

The other four either have the normal number of chromosomes or one fused one. By chance, three of these four die before giving birth and the fourth goes off to seek his fortune. Now the only two who are left have two fused chromosomes. As will all of their children.

This scenario has the idea of genetic drift in it. Because of the small population, chance can have a huge effect on the tribe. Certain traits die out because the few people who have the trait die or do not have children. Or happen not to pass the trait down.

Getting back to our story, everyone in the tribe now has two fused chromosomes. They have 23 pairs of chromosomes instead of 24. And because they still have all the same information, they are perfectly fine!

I want to emphasize again that we don't know for sure if this is how it happened. This is just an example of how it might have happened.

And it isn't just something we're making up here. We can see examples of changes in chromosome number happening around us in nature right now.

For example, wild horses have 33 pairs of chromosomes and domesticated ones have 32. This was a relatively recent change that happened because of the sort of translocation we are talking about. In fact, this happened recently enough that these animals can mate and have fertile horses.

Butterflies are also another example. There are many very closely related species of butterflies that have large differences in chromosome number. Take the South American Philaethria butterflies. Their chromosome number can range anywhere from 12 to 88! The butterflies are fine because they all have the same information. It's just arranged in different ways.

But scientists aren't sure yet if these changes actually cause two species to split. They could also have happened after the species split. In which case, chromosome number didn't have anything to do with making a new species.

And the same thing is true with humans. We still aren't sure the jump from 24 to 23 chromosomes is what makes us human. Like I said, it is certainly possible that we split from our ape ancestors before the difference in chromosome number. So in other words, humans may have become a new species for reasons other than chromosome number.

What is it that makes us human?

We don't yet know what specific differences at the DNA level are responsible for making us different than apes. But scientists have a few ideas. And many of these ideas revolve around changes in our DNA that gave our human ancestors some advantage.

There actually have been quite a few rearrangements between human and ape chromosomes over the years. It may be that one of these caused new information to be created. Or some to be lost.

This does happen sometimes. Remember, I said the information is in discrete chunks. A lot of this information is found in our genes.

If something changes one of these genes so it gets used at the wrong time or in the wrong place, then it will change something about that animal. For example, if a part of a chromosome gets flipped right in the middle of a gene, then that gene will be affected.

Mixing up chromosomes sometimes causes these kinds of changes. And if that change has an advantage, then it will spread.

Another idea is that small changes in DNA sequences (mutations) could have caused a gene to change. Like maybe a gene for brain size or talking. Or it could be some combination of many DNA changes.

Scientists have discovered several possible candidate genes for making us human. While the vast majority of our DNA is the exact same as chimpanzees, there have to differences too. If they were exactly the same, then we wouldn't be 2 different species. Finding differences between human and chimp DNA is actually a very big area of research for scientists. They hope that by studying these differences, we can figure out what makes us human.

Unfortunately we can't go back in time and figure out exactly what happened. But studying our DNA is the best way we can retrace history.

Monica Rodriguez

Ask a Geneticist

More Information