A team of scientists led by Andreas Hejnol from the University of Bergen published an article in the journal Nature that questioned the truth, which until recently seemed absolutely unshakable. Researchers questioned that the central nervous system is inherited by all of its owners from a common ancestor. To this conclusion biologists pushed the study of strange sea creatures.
Recall that a class, genus, species or any other community in biological taxonomy is called a taxon. Bilateria is a rather unusual taxon. It combines flies with crocodiles and people with worms. One of the features characteristic, as recently believed, only for its representatives is the presence of a nerve cord (nerve cord), which can be abdominal or dorsal. Some worms have two nerve cord, but in humans it is called the spinal cord.
Using morphological, paleontological, genetic and other evidence, biologists have long come to the conclusion that all bilateria originated from a single ancestor. This unimaginable common ancestor of people and flies and handed us “inherited” the central nervous system, which, it turns out, arose in evolutionary history once.
However, the authors of a recent study call into question this established truth. To make such a bold conclusion, scientists had to travel by boat on the fjords of Norway and Sweden, and also to moor to rocky islets off the coast of the United States. Scientists sifted raised from the bottom of the mud or even examined the intestines of sea cucumbers to find parasites in them.
The result was worth it. A lot of strange creatures got into the network of researchers. What was worth only representatives of the type Xenacoelomorpha! The species Xenoturbella bocki does not have a nerve cord. His nervous system is similar to that of jellyfish and resembles a network of nerves. But the species Isodiametra pulchra has as many as eight nerve cords. Kind Meara stichopi can boast exactly one neural cord, and even running on the back, as in vertebrates. No less diverse “technical solutions” were demonstrated by representatives of Rotifera, Nemertea, Brachiopoda and Annelida types.
So, there are animals that do not belong to bilateria, but, nevertheless, they have various nerve cords. At the same time, some of their “neighbors by type” can not boast of anything like that. Already these purely anatomical data, according to the Heinol group, refute the idea that this “innovation” appeared in evolutionary history once and only in the ancestors of bilateria.
And what about the genetic data? It turns out that not everything is going well with them. As reported in the research press release, scientists focused on the activity of the gene bmp, which is considered inherited from the common ancestor of bilateria. Previous studies on various animals, from fruit flies to sea worms, have shown that it is responsible for the onset of development in the embryo of the central nervous system.
True, the journal Nature specifies that this function was called into question in 2006. Then another scientific group discovered that in some species of worm this gene is activated long before the development of the nervous system begins.
But then it was considered that these worms are an exception to the rules. Somehow, biologists decided, the gene acquired additional functions from them. In the end, they have two nerve cords, among the bilaterians it is exotic.
But the work of Heinola and colleagues again raised the issue of the role of the gene bmp. The scientists found that this gene “turns on” (as experts say, is expressed) much earlier than the development of the nervous system begins in their embryonic development. Moreover, when researchers blocked the work of this gene, the nervous cords of strange sea creatures evolved as if nothing had happened!
All this, the authors conclude, suggests that the central nervous system has appeared in evolutionary history many times and independently.
By the way, the same can be said about multicellularity, the mineral skeleton and the achievement of macroscopic dimensions. And this list is still incomplete. Independent branches of life with amazing consistency come to the same optimal for this planet evolutionary solutions.