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.
Evolution is the change in organisms over time that gives rise to new species. Development is the process by which a fertilized egg, or embryo, generates the cells, tissues, and organs of a new individual and assembles them into their proper form. Evolution produces the body shapes of the animal kingdom; development produces the body plan of individuals.
Biologists have been making connections between these two processes since the 19th century. But in the last decade, these studies have intensified and even spawned a new field of study: evolutionary developmental biology, or, as it's often known, "evo-devo.'' Using new techniques of biology and genetics, researchers are now investigating development at the molecular level, the genes that regulate and orchestrate the unfolding of a new life. Moreover, the genes not only serve as a construction and operating manual, they also contain a record of the evolutionary history of the organism, because many of the same genes were used by direct ancestors. "Evo-devo'' researchers investigate the ways that evolution has modified embryological processes, and, conversely, how developmental mechanisms have influenced evolution.
Even before Darwin, biologists recognized that species that looked quite different as adults often had close similarities as developing embryos. Many four-legged animals go through embryonic stages that have similar features -- gill arches, a notochord, segmentation, and paddle-like limb buds -- as they develop into different adults. To Darwin, the embryonic resemblances were strong support for the theory of evolution.
One of Darwin's contemporaries, German biologist Ernst Haeckel, summed up the argument in a famous, pithy statement: "Ontogeny recapitulates phylogeny.'' That is to say, in the process of development, an individual passes through the adult forms of all its ancestors. So, Haeckel proposed, by examining the development of an embryo you could read its entire evolutionary history in the transition from one stage to another. In fact, this isn't strictly true, and the drawings Haeckel made exaggerated the embryonic similarities between species.
But biology now has new tools, from microphotography to molecular biology, with which to examine the process of development in embryos. These new tools reveal that different descendants of a common ancestor do indeed usually go through embryonic stages that resemble each other and their common ancestor The processes that guide embryonic development are conserved by evolution and reused again and again.