Evidence of evolution: embryology | Evolution | Middle school biology | Khan Academy

• Do you ever wish that you had a tail? You could swing your way to school, bake pies more efficiently, and carry an umbrella while keeping your hands free.

The funny thing is, you did have a tail once, before you were born. Back then, you were an embryo. An embryo is an organism that is in the earliest stages of development before it is born or hatched. Early on in the growth of a human embryo, the embryo has a tail-like structure.

As time goes on, the embryo grows, and eventually the cells that made up that tail structure shift and form the tailbone, which makes up the bottom of the spine. By the time the embryo is eight weeks old, the tail is not visible at all.

Humans are not the only species to have tails as embryos. We share this trait with the embryos of many other vertebrates, which are animals with a backbone, such as monkeys, mice, turtles, and chickens. Scientists call features such as embryo tails homologous features.

Structurally similar anatomical features that two species share that indicate that the species share a common ancestor. Identifying homologous features can help scientists figure out how different species are related to each other and how they evolved. Studying embryos is a helpful way for scientists to find similarities between species.

Similarities that might not be visible once the animals are born and grow up. In general, embryos of related species have more obvious homologous features at earlier stages of development, before the embryos' anatomy becomes highly specialized.

During the stages of an embryo's development, the embryo goes through a lot of physical changes. The embryo of an elephant starts out weighing less than a gram and it eventually grows to its birth weight of about 100 kilograms.

Let's take a closer look at the kinds of changes the elephant embryo goes through as it develops. As an embryo grows, its physical structures change. Some structures become visible, and others disappear. For example, towards the beginning of development, an elephant embryo has structures called pharyngeal arches, or gill arches, on its neck.

As the embryo grows, the pharyngeal arches change structure and help form the ears and jaws of the elephant. And it turns out, all vertebrate embryos have pharyngeal arches early in their development.

In fish, these arches develop into gill structures. In humans and other mammals, these arches develop into ear and jaw structures, just like they did in the elephant. Pharyngeal arches are homologous features, even though we can only see this homologous feature early on in embryo development.

Pharyngeal arches provide evidence to scientists that all vertebrates share a common ancestor. More distantly related species tend to share fewer homologous features during both embryo development and after birth.

More closely related species tend to share more homologous features during both embryo development and after birth. So even though you don't have a tail, as far as I know, studying homologous features in embryos shows scientists that humans are related to many tail-using creatures because of evolution.