Embryos Show All Animals Share Ancient Genes

The discovery helps to explain why human embryos resemble those of animals at the early stages of life.

THE GIST

Embryos for humans and other animals often look alike at certain developmental stages because they share ancient genes.

These ancient genes are expressed during a middle "phylotypic period" of embryonic development for all species.

Developing human, fish and other embryos therefore at times share features, such as tails and gill-like structures.

Human embryos resemble those of many other species because all animals carry very ancient genes. These genes date back to the origin of cells, which are expressed during a middle phase of embryonic development, according to two separate papers published in this week's Nature.

The findings help to explain why our embryos have a tail when they are a few weeks old and why human embryos retain other characteristics, such as fur-like hair and fish embryo similarities, seen in the developmental stages of other species.

"On average, the similarities will be even stronger for more closely related species," Diethard Tautz told Discovery News.

"However, it is indeed true that even fish and human embryos go through a phase that looks very comparable, while they are rather different before and after this," added Tautz, who co-authored one of the papers and serves as managing director of the Max Planck Institute for Evolutionary Biology.

He and colleague Tomislav Domazet-Loso tackled the "ontogeny recapitulates phylogeny" puzzle. This expression means that a more advanced organism, like humans, will resemble less advanced species during it's development stages.

The researchers studied the genes of zebrafish. The scientists classified the genes according to their evolutionary origin in the history of life and measured their contributions at different time points in the zebrafish life cycle.

The researchers found that the oldest genes evolutionarily were expressed during a middle phase, also known as the "phylotypic period," of the zebrafish's embryonic development.

"(Our) paper shows in addition the interesting effect that old animals return also to old gene expression patterns, suggesting that they increasingly switch off the genes that have helped them to go through adulthood," Tautz said.

For the second study, Casey Bergman, a lecturer in computational and evolutionary biology at the University of Manchester, Pavel Tomancak and their colleagues approached the embryo similarities' puzzle from another perspective: They measured differences in gene expression between various species of the fruit fly Drosophila.

Again the scientists observed that development among the various species was comparable during the middle phylotypic phase.

"Genes that are active during the middle embryonic period are involved in organizing the overall body plan of the organism, such as the body axes and major tissues and organs," Bergman explained to Discovery News.

He continued that the earlier and later developmental stages instead "use genes involved in utilizing materials in the egg provided by the mother and more species specific aspects of animal form involved in environmental adaptations."

Evolutionist Charles Darwin noticed such patterns in other species, but the two new studies show what is happening at the genetic level. They all support what is known as the "hourglass model" of embryogenesis.

It's dubbed that because the middle point marks the often-shared phylotypic period when the individual's basic body plan is laid down, while the beginning and end points are more genetically divergent and unique to the particular species.

"The similarity of animals at the center of the hourglass is shared by species in the same group of organisms, that is, among all vertebrates (including mammals, fish and birds) or among all insects, but not between insects and vertebrates," explained Bergman.

Human embryos at a certain stage therefore have a tail and folded neck structures that, in fish, later turn into gills. In humans, the folded structures become our jaws. Bergman said human embryos also possess a laryngeal nerve that travels from the brain underneath the aorta and then back to the larynx.

"The unnecessarily long path of this nerve is shared by all vertebrates and only makes sense when considering the origin of vertebrates is from a fish-like ancestor," he said.

"We are just very highly evolved fish!" Bergman concluded.