Testicles are an interesting subject of debate among scientists, since they don’t know if they first ascended or descended among mammals. However, researchers managed to explain the evolution of descending testicles by using DNA from elephants and other mammals that keep their organs inside.
Which were the first: ascending or descending testicles?
Elephants, as well as manatees or several rodent species, do not have descending testicles. Their organs lie inside of their bodies, so researchers decided to choose them as their object of study. This way, they hoped they could find out if ascending or descending testicles came first.
Humans, as well as most mammals, have their testicles hanging in a scrotum. If the organs stayed inside the body, they would overheat, thus affecting the sperm. However, Afrotherians, a mammal group from Africa, keep their organs inside. This unusual trait left researchers baffled, wondering if they inherited the trait or evolved it later on as a biological oddity.
One solution could have been to study ancient DNA, but it’s often hard to pick it up from disintegrating tissues. However, researchers discovered a new solution that uses modern DNA to build an image of ancient genetic material. Therefore, they selected 71 mammals with placenta and looked at their genetic material. Then, they tried to identify a common ancestor and look for traits for either ascending or descending testicles.
Afrotherians evolved testicles inside the body only later
There are two genes that trigger the evolution of hanging testicles, so researchers looked for those genes in particular. This way, they found out that Afrotherians lost these genes around 100 million years ago. The evolutionary moment coincides with the time when these creatures split from the common ancestor.
Researchers had selected four different Afrotherian species, including the elephant, and each of them had evolved the ascending genes as a result of different genetic mutations. This showed these genetic changes occurred more recently. Therefore, researchers solved the mystery, showing the common ancestor had descending testicles.
The study was published in the journal PLOS Biology.
Image source: Wikimedia Commons