It might seem like a joke, but it is not. A new study published on Nature It shows that the fingers of hands and feet could genetically derive from the cloaca of ancient fish, that is, from that unique opening which still serves today serves to expel urine, feces and reproductive cells.
This is supported by a group of scientists from the University of Geneva, the Federal Polytechnic of Lausanne (EPFL) and various US institutes. The study analyzed the genetic activity of mice and zebrafish, two animal models often used in research, and has shown that the same region of DNA which in mammals guides the formation of fingers is active, in fish, in the formation of the cloaca.
The researchers explain that it is an example of genetic reuse by evolution: instead of creating new regulation systems from scratch, evolution would have “recycled” the existing ones, adapting them to new structures.
A genetic code that comes from the cloaca and reaches the fingers
For some time now it was known that Hox genes, in particular those of the Hoxd group, are essential to organize the development of the body in animals. In this case, scientists have observed the behavior of Hoxd13, a gene that is activated in the mice in the early stages of the development of the limbs and plays a key role in the formation of the fingers.
The team then examined the same gene in Zebrafish, a fish that does not have fingers, but that has the same Hox sequences in its genome. The results showed that Hoxd13 in the fish does not turn on in the fins, as we could have expected, but in the region that will form the cloaca.
To understand if the two functions were connected, the researchers eliminated, using Crispr, the part of DNA that regulates the activation of Hoxd13. In mice, this change has blocked the development of the fingers. In fish, he had no effect on the fins, but prevented the cloaca from forming correctly.
According to the authors of the study, this means that the genetic mechanism that today serves to form the fingers was born for a completely different purpose: to give rise to an excretory opening in the ancestors of the vertebrates.
Evolution for recycling: nothing comes from scratch, everything turns off
The research group, coordinated by Denis Duboule, underlines that evolution often does not build complex structures from scratch. In many cases, it reuses existing systems, moving them to different contexts. This process is called evolutionary co -optation.
In the case of the fingers, the evolution would have taken a genetic system already active in a terminal part of the body – the cloaca – and would have employed it, millions of years later, to form the ends of the limbs in the terrestrial vertebrates.
According to the researchers, the regulatory landscape that today rules Hoxd13 is much older than the fingers themselves: it dates back to before the appearance of the limbs and also of the external genital organs. Today, this same region of the genome is still active in multiple areas of the body in mammals, for example in the urogenital breast, an embryonic structure that, in the early stages of development, recalls the cloaca.
The anus could also be born from another opening
In parallel to this study, other researchers are carrying out a similar hypothesis. Analyzing Xenoturbella bockia marine worm considered very primitive, they observed that the genes associated with the formation of the anus in more complex animals are activated around the gonoporo – the opening through which these organisms release reproductive cells.
According to the evolutionist Andreas Hejnol, of the University of Bergen, it would be another example of evolutionary reuse. In other words, a structure born for reproductive use may have been adapted subsequently to an excretory purpose.
The transition from fins to the limbs is considered one of the most significant passages in the history of life on earth. This study adds an unpublished piece: the genetic part that allowed this transformation was already present, but it served anything else.
Understanding how evolution reuses the available genetic material helps to clarify how new anatomical structures arise, but also to recognize how much the links between the different parts of the body and their functions are often deeper and complex than it seems.
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