Scientists look for clues in our own DNA that tell stories about the evolution of our prehistoric animal ancestors. Inside the human genome, we can see remnants of the genetic code for constructing a yolk — a protein-rich food sac for embryos that develop inside eggs — even though yolks are normally associated with egg development, as seen with birds or reptiles. As a result of these genes, humans grow a tiny yolk sac during early development (although there's actually no usable yolk inside), before the organ disappears. The yolk is a trait that has been passed down in our DNA, but that humans no longer need to grow. Scientists see this yolk fragment as clear evidence that humans evolved from an egg-laying animal.