Origin of jaws of vertebrates from gills is revealed

(ORDO NEWS) — Scientists at the University of Southern California and the University of Cambridge have uncovered the origin of jaws in vertebrates. According to a study published in the journal eLife, they originated from the gills of ancient fish.

In fish, jaws share a common embryonic origin with gills. During development, jaws and gills arise from embryonic structures called gill arches.

The first of these arches is called the mandibular arch, because it gives rise to the jaws, and additional arches develop into the gills.

There are also anatomical similarities: the gills are supported by upper and lower bones, which can be considered analogous to the upper and lower jaws. However, the hypothesis that the jaw evolved from gills is still controversial.

Almost all fish have a tiny anatomical structure called a pseudobranchia that resembles a vestigial gill. However, the embryonic origin of this structure has been uncertain.

The scientists used imaging techniques to trace the fate of embryonic cells in zebrafish and showed that pseudobranchia originates from the mandibular arch, which gives rise to the jaw.

In addition, many genes and regulatory mechanisms govern the development of both pseudobranchia and gills.

According to the scientists, this means that the structures that arise from the mandibular arch – pseudobranchia and jaw – could originate from the gills, which were changed over the course of a long evolution.

Rays, belonging to a completely different evolutionary class of jawed vertebrates (cartilaginous fish), are also known to have pseudobranchia.

Thus, pseudobranchia were present in the last common ancestor of all jawed vertebrates. Together, this provides evidence for the classical theory that the gill structure evolved into the vertebrate jaw.

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