Early development
From week 8 of gestation, the feet and hands begin to form, first as small paddles and then, around 11-14 weeks, nails appear, as well as lines on the palms. Their anatomy is very complex and an essential part of humans since the exterior is physically manipulated. The tips of the fingers are one of the regions with the most nerve endings in the human body; for this reason, the hands are closely related to the sense of touch as they are the main source of information from the outside.
Awesome anatomy
The fingers are assigned a name that we learn from a very young age (although they vary in different places). If we see our hand with the palm up and from the outside in, we will first find the "thumb" or also called the "big toe"; then the "index" finger; then the "middle finger," also called "middle finger," "older," "friendly" or, of course, "rude." Then we have the "ring finger" (so-called because in Western culture, it is where the wedding ring is worn) and the "little finger" or "little finger. The hemisphere controls each hand on the opposite side of the body, and there is always one dominant over the other. It is estimated that at least 10% of the world's population is left-handed, meaning that they live with the left hand's bad reputation.
The part attached to the wrist is called proximal, and the opposite part distal. We know the palm or palmar well, while the part opposite it is called the dorsal. Human hands have 27 bones, 8 in the carpus, 5 metacarpals, and 14 phalanges. There are 3 types of phalanges, the distal, the middle, and the proximal; the extraordinary thumb has only two phalanxes, no middle phalanx. The fingers do not have muscles; they move thanks to the tendons and muscles of the palms.
Why humans have five fingers? Why not seven?
Human hands look very different from the little hands of a bat or the large fins. But, as Charles Darwin made clear with the beaks of the finches, both beaks and limbs are different today but at the same time have the same origin. This results from so-called divergent evolution, a mechanism by which the same biological structure undergoes modifications in several species as organisms adapt to new environments. This is why primates, whales, and bats share the same pentadactyl limb, also known as iridium, but at the same time, the reason why they look so different.
Although this is clear, the truth is that scientists still have much to learn to understand how these members evolved or what genes prevent humans from having seven-fingered hands like mice, for example. In this sense, an investigation published this Wednesday in "Nature" has discovered two genes that can close this gap a little because they regulate the formation of the hand and at the same time the development of the fins of the rays. They are called hoxa13 and hoxd13.
"This result is exciting because it establishes a clear molecular link between striped fins and fingers," explained Yacine Kherdjemil, the first author of the study and a scientist at the Montreal Institute for Clinical Research (IRCM), in a statement.
This is really interesting because the fossils found so far show that the current creatures' ancestors were polydactyls, with seven phalanges, and not pentadactyl. Can hoxa13 and hoxd13 be the key to figuring out how that change happened?
Hox genes
In nature, such drastic effects, such as going from five fingers to seven, are not so strange. Small changes in structural genes, or hox genes, cause large changes during embryonic development and the so-called body plan (" bauplan " in German). If these genes had to build a building, they would be in charge of deciding the rooms or plants; therefore, small changes in them cause important alterations in the resulting structure.
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