The Best-Laid (Body) Plans
Humans, indeed all tetrapods, have 1 head, 4 limbs arranged in two pairs, a tail, and a variety of other gross morphological features. Why? And more interestingly, where did this basic body plan come from?
In fact, the simple fact that all tetrapods have the exact same body plan is a huge indicator for common descent. Whales, snakes, dolphins, skinks, all have the remnants of the 4 limbs, even if they aren’t commonly present. This is called an atavism and is additional evidence of common descent.
But we’re talking about body plans and why tetrapods have the features that they do. Why do we have a top and bottom, front and back, left and right? The answer is simple… all organisms have these features and the genetic systems that control the development of these features in even the most primitive animal do the same thing in our bodies.
During the first few days after conception, the single cell that will eventually form a fully adult human divides several times. First into two cells, then four. Eventually forming a small, hollow ball of cells called a blastocyst. On the sixth day, the blastocyst embeds itself into the lining of the uterus.
I learned something that I never realized before. Human anatomy and reproduction is not my favorite subject of study (feel free to insert juvenile ‘practice vs. theory’ joke here) and I just hadn’t paid very much attention. But that blastocyst, will not become a person. The blastocyst becomes the placenta. A tiny group cells where the blastocycst implants in the uterus will become the person. So far to this point, the zygote has been busy making itself a structure that it can survive in, not as much attention has been paid to the organism itself yet.
But now that it has implanted… now the cells that will become an individual start ramping up their activity.
Every animal on this planet is basically a hollow tube with three layers of cells in the tube walls. The hollow tube becomes the animal’s digestive system. Mouth at one end, anus at the other, and some number and variety of tubes in the middle.
Everything else comes from those three layers of cells.
But what is really interesting is that the same parts of the cell layer form the same parts in every animal. One end of the clump of cells (due to the concentration of some chemicals) becomes the head end and the other (without that concentration of chemicals) becomes the tail end. The cells themselves form these chemicals. A different chemical signal tells the cells whether they are the back or the front (dorsal or ventral) of the organism. Once you have a front and a back and a head and a tail, you also know which side is left and which is right.
The same genes, produce the same chemicals in every animal. It has been shown for almost a hundred years that chemicals extract from one organism can cause the correct formation in another organism. Modern techniques expand on this by replacing one organisms’ gene (say a frog) with another organisms’ gene (say a sea anemone) to see if the frog will develop correctly. It does.
It can go even farther than that. We can find homologous Hox genes in humans and fruit flies. That’s one reason fruit flies are so useful to scientists. They develop in exactly the same way as all other animals, even humans.
And, yes, when you study them in detail, you discover that sea anemones do have sides and a front and back. Even though they are radially symmetrical (humans and most other animals are bilaterally symmetrical), they still have a head end and a tail end. Those same genes that form the head and tail ends are in other animals, including humans.
So, the body plans that we have been in place since animals existed (and probably before, as we will discover in chapter 7).