One of the greatest mysteries of evolution is the emergence of heads and skulls in vertebrates. The authors of the new study claim to have solved this problem by carefully examining the embryonic development of the Japanese or Pacific lamprey, one of the most primitive modern vertebrates.

Vertebrates play a special role in the modern biosphere. They are large, intricately organized and belong to the most evolutionarily successful animals. Vertebrates are creatures that have a backbone and belong to the chordate class. It also includes some other animals such as lancets and tunicates. They do not have a spine, but they do have another supporting structure, the chordae, that runs along the body for at least part of their lives.

The first ancestors of chordates appeared during the Cambrian period, at the dawn of the modern stage of the evolution of the biosphere. They are thought to resemble lancelets, small modern chordates with neither spine nor head; There is only a tail attached to the body.

Soon the first vertebrates appeared – most likely, they were associated with the rotators around the clock, primarily the testicles. These animals have only a rudimentary skull and no jaws, which does not prevent them from entering the body of fish and parasitizing them. One big question remained: When did vertebrates develop heads, that is, have a separate head part of the body and a corresponding part of the skeleton?

Some scientists believed that the head of the first vertebrate (and therefore its descendants) consisted of modified vertebrae, that is, it was originally part of the spine. Goethe was one of the first to express such an assumption. Others argued that the skull arose independently and independently of some other structure.

Japanese researchers who published articles in the journal iScienceHe claims to have found the answer to this question. Using a series of ultrathin sections and scanning electron microscopy, they examined in detail the embryonic development of the Japanese or Pacific sea lamprey. Letenteron camtschaticum. Previously, there was no such detailed and reliable description of lamprey embryogenesis. The researchers also examined gene expression patterns. L. camchaticumand for comparison – lanceolate and semichordates (invertebrate animals close to the chordate type).

The development of the mesoderm of the lamprey’s head and the somites was the focus of close attention of scientists. This is the name of the paired mesodermal formations located symmetrically on the sides of the chord and in the germinal nervous system of vertebrates.

Previously, embryologists had noticed that in the main part of vertebrate embryos there are also paired formations, which are bubbles that resemble somites. It was necessary to find out whether these and other structures really have a common origin. The authors of the new article confidently denied such a connection.

They confirmed that characteristic clusters (clusters) of cells appeared in the head of the lamprey embryo. However, they do not have the rosette shape (circles with a hole in the middle) that most vertebrate somites take. Such nests are important in later stages of embryo development. Moreover, in the tissues of the developing lamprey, the activity of genes involved in the formation of somites and the division of the embryo into similar areas located one after the other was not detected.

It turns out that the mesoderm of the head of vertebrates is developed already in the early stages of their evolution. At the same time, according to the study’s authors, somites most likely arose from an older common ancestor of the latter, a large group of animals that includes chordates (including vertebrates), hemichordates, and echinoderms.

The scientists believe their paper will put an end to a century and a half-long debate among zoologists about whether the head of vertebrates is “on its own” or simply a derivative of their spine.