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• The 23^rd chromosome base pair contains what are known as our sex chromosomes. For most females, this pair contains two X chromosomes, while for most males, it contains one X chromosome and one Y chromosome.

• Since 2010, scientists have known that the Y chromosome is rapidly evolving in humans, but a new study shows that the same can be said across all Great Apes—the closest relatives to humans.

• Surprisingly, the Y chromosome shows immense variability even between species of the same genus, such as chimpanzees and bonobos.

Most animals belonging to the hominid species known as Homo sapiens have twenty-three pairs of chromosomes. Barring any specific chromosomal conditions, these pairs are shared similarly among all humans, except for the 23^rd base pair. This base pair, known collectively as the sex chromosomes, carries the genes that express biological sex. In most females, sex is expressed as two X chromosomes, and in most males, it’s expressed with one X and one Y.

Since 2010, scientists have known that the human Y chromosome has been rapidly evolving—a surprising finding, considering that evolutionary biologists thought the Y chromosome was stagnant for years. Now, a new study from the Penn State, the National Human Genome Research Institute, and the University of Washington shows that the rapid changes are occurring in not just the human Y chromosome, but also in the Y chromosomes of other Great Ape species, including chimpanzees, bonobos, western lowland gorillas, the Bornean and Sumatran orangutans, and the more distantly related siamang gibbons.

Using what’s called telomere-to-telomere (T2T) sequencing (telomeres are protein structures that serve as the ‘caps’ of chromosomes), the researchers used computing software to make comparisons among the species, and discern what parts of the chromosome changed and what parts remained the same. Among the six species, the Y chromosome exhibited much more variability than the X chromosome. For example, the X chromosomes of humans and chimpanzees are about 98 percent identical in makeup. On the other hand, only about one-third of the Y chromosomes of the two species matched. The results of the study were published in late May in the journal Nature.

“Sex chromosomes started like any other chromosome pair,” Penn State’s Kateryna Makova said in a press statement, “but the Y has been unique in accumulating many deletions, other mutations and repetitive elements because it does not exchange genetic information with other chromosomes over most of its length.”

Of course, this doesn’t mean individuals with this chromosome are somehow more evolved that those without it—after all, we’re all members of the same species. But even among species in the same genus, such as bonobos and chimpanzees (who likely separated only 1 to 2 million years ago), differences between Y chromosomes were stark.

It’s worth noting that these variations are derived from just one individual. Like humans, there’s likely much more variation within the species as well.

This research gives a glimpse into the evolutionary pressures placed on the Y chromosome. It also showcases, according to Makova, what is known as “male mutation bias”—how sperm production involves much more replication than egg production. Anytime you have a lot of replication, you also increase the chances of mutation.

While understanding these differences among the Y chromosomes of Great Apes will likely be a boon for human health, it could also help us understand our endangered primate cousins.

“It’s important to remember that these great ape species are all endangered,” Makova said in another press release. “Not only can we learn about human evolution from these sequences, but we can apply what we know about their genomes and human genomes to better understand the biology and reproduction of these endangered species.”

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