3.8: How to become an animal
Three driving forces of eukaryotic evolution:
- Prey and predator interactions
- Surface and volume
- Ecological pyramid (Fig. 3.8.1).
finally resulted in appearance of animals and plants. But these two groups originated differently.
Animals belong to the highest level of the pyramid of life (Fig. 3.8.2). They not only multi-cellular but also multi-tissued creatures. It is easy to become multicellular, enough is not to split cells completely after mitosis. And the big advantage is immediately feel: size.
To make a big body, it is much easier to join several cells then grow one cell. Explanation lays (as well as explanation of many other biology phenomena) in the surface/volume paradox : the more is the volume, the less is relative surface, and this is often bad. Multiplication of cells allows to be big without decreasing relative surface. And being big is a good idea for many living things, especially for plants (the more is the size, the more intensive is photosynthesis) and for prey in general (the bigger is prey, the more chances to survive after contact with predator).
But this is not working out of the box for active hunters like animals’ ancestors! To move, they need also the tight coordination between cells, and to eat, they need altruistic cells which feed other cells. Consequently, first animals (like phagocytella : Fig. 3.9.1) must acquire have at least two tissues: (1) surface cells, adapted to motion, likely flagellate, and (2) cells located in deeper layers, adapted to digestion, probably amoeba-like. This evolution could go through several stages:
- Blastaea: not the animal yet. Volvox , Proterospongia .
- Phagocytella. Two tissues: kinoblast and phagocytoblast . Trichoplax .
- Gastraea (jellyfish without “bells and whistles”). Three tissues: ectoderm, entoderm and mesoderm. Closed gut.
What about communication, circulation, gas exchange etc.? If the first animal was small enough, all of these will run without specialized tissues, via diffusion, cell contacts and so on. But when size grows, the surface/volume paradox dictates that new and new tissue and organ appear. And size surely will grow because there is a constant race of arms between prey and predator, and between different predators.