It’s pretty amazing that you and I started our lives as blastulas. In the early days following our conception, we were little more than a ball of cells, rapidly dividing according to the instructions in our genetic code (found in the nucleus of every one of those cells). As a consequence of our shared evolutionary history, all vertebrates begin their lives this way; our genetic code is comprised of thousands of nucleotide sequences, each containing a set of instructions, “build this protein, build that protein, etc.”, and the cells keep replicating and replicating. Eventually, just like building a massive spaceship or castle out of legos, a structure that has simple beginnings gets more and more complex, and the differences that we observe between species start to manifest themselves in development according to each species’ unique DNA blueprints.
Here are some photos that I took of embryogenesis in the Wood Frog (Rana sylvatica). In the first picture you see the blastula, a little ball of cells that is growing and growing as the cells continuously divide. Eventually, the blastula starts to fold inward (i.e., gastrulation) and we start to see the bilaterally symmetrical body plan take shape. The white stuff at the bottom of the second picture is the part that will eventually turn into the frog’s anus. The embryo keeps growing and begins to elongate. In the third picture in the sequence (center left), we see the head begin to differentiate. The frog’s brain is now beginning to form (upper right of the embryo in the center left pic), and the central nervous system and other vital organs are all developing as per the instructions found in the genetic code. The very same process happened to you and I in the earliest days of our lives.
I went away for a few days, and when I returned the tiny embryo had developed into a healthy tadpole (bottom picture). Eventually, as per the instructions in its genetic code, the animal grew legs and transitioned into a terrestrial lifeform as an adult Wood Frog (center right). Next year, this frog (if it survives) will find a mate and start the reproductive cycle all over again, just like its ancestors have done for millions of years. We have been doing the same thing, and if you trace our family trees back far enough, they eventually converge. That’s right, this frog and I share a common ancestor. It is one of the most beautiful and profound truths of nature, and for me, a source of deep meaning. The proof is right there in our shared genetic code, and in the highly conserved early stages of embryonic development.