How animals develop from an embryo
What Does a Tree Sloth and a Crab Have in Common?
A thought occurred to me as I was sitting on the tube; have you noticed that nearly all vertebrates (animals with backbones) have the same body plan? I mean they all have a head, two front legs (or arms) and two legs. This seems rather incredible if you consider the diversity of the animal kingdom. If we just take mammals alone, they have adapted to run (cheetah), fly (bat), swim (whale), and even dig (mole). So why do they all share the same basic body plan? Were they all designed separately or did they evolve from a single ancestor?
If we simply look at the bones in the forelimb of each of these creatures we can instantly see the same pattern. Each has five digits, a set of small wrist bones, two bones in the forearm, a single humerus, a shoulder blade and collar bone. The shapes of the bones are subtly altered to suit the particular environment in which the animal lives.
The mole, for example, has digits that are short and stout, designed for slow but hard work. In contrast, the bones of the cheetah, or gazelle, are light and slender for chasing, or getting away, respectively. In the nineteenth century, E. Geoffroy Saint-Hilaire, a respected biologist, compared the body plans of vertebrates and crustaceans (such as lobsters, crabs and so on) and suggested that the body plan of these animals were similar to those of vertebrates but upside down! He suggested this because the spinal cord of crustaceans sits below the gullet, whereas it sits above the gullet in vertebrates. He was laughed at and his theories rubbished because everyone knows that crabs and people are entirely different. Aren't they?
Modern embryologists study the pattern of genes in the developing embryo to try and understand how the activation of these genes leads to actual physical development. This is much like trying to piece together the instruction manual for an MFI wardrobe by looking at the separate parts of the wardrobe and comparing them to the pictures in the instructions.
The hard bit is to try and get those pictures in the right order so that they make sense. By studying the expression of the genes in many different types of animals, we have found that the same genes are switched on in a similar pattern in animals as diverse as fruit flies, lobsters, chicks and humans. Further, the activation of certain genes has backed up the ideas of Geoffroy by showing that crustaceans develop "upside-down" to vertebrates. So the tree sloth and crab do have something in common after all...they both live upside down...!