Horses have only one finger on each paw, the hooves being the equivalent of the phalanx of our middle fingers. However, their ancestors had more fingers on each paw, ranging from three to four.
A new study indicates that, during the evolution of this animal, the middle finger grew in such a way that it formed what today we call a hoof. According to the researchers, all that remains of the other fingers are the two small stumps next to the hooves, known as metapodials.
A recent investigation, led by Kathryn Kavanagh of the University of Massachusetts Dartmouth, revealed that horses do not always have just one finger.
“The evolution of the modern horse limb, and in particular the reduction in the number of fingers during the evolution of the lineage, is one of the most iconic evolutionary transition stories documented in the fossil record,” wrote the study authors.
Horse embryos have five fingers
About four weeks after conception, horse embryos have five fingers, just like humans. In the following weeks, the middle finger increases, while the outer pairs merge and shrink to form the metapodials. When the embryo reaches two months, only the middle finger is visible.
Several animals have 5 fingers in the embryonic phase
The scientific article, recently published in the Royal Society, also indicates that horses are not the only ones: from pigs to crocodiles, everyone started their lives with four or five fingers.
What happens is that many ended up with fewer fingers, either by fusion or because certain fingers stopped growing, or some other mechanism.
Until now, scientists thought that genetic factors would determine how many fingers an animal has.
However, this new study suggests that all animals have four or five fingers – only later, during their evolution and development, the number of fingers is reduced.