7 - Skeletons and Life

Summary

Here, I deal with the issue of how organisms support themselves against the downward pull of gravity. First, I look at the microscopic skeletons of cells, which help both to maintain the shape of a static cell and to alter that of a mobile one. Then I consider the higher-level skeletons that are needed by large multicellular creatures, whether animals or plants. In the animal kingdom, I focus on the solutions to this issue that we find in the three largest phyla. Vertebrates and arthropods have solved the problem of support in what might be called opposite ways – skeletons on the inside and outside respectively (endo- and exo-skeletons). Molluscs are harder to generalize about. An external shell is the commonest hard structure, but in some groups the shell is reduced and internal. Shells are different from endo- and exo-skeletons in that they have to be carried, so are a mixed blessing from a gravitational perspective. Outside the three main animal groups, other skeletal solutions are found. These range from the hydrostatic skeleton of worms to the woody skeleton of trees. Wood has allowed the evolution of the tallest organisms on the planet – coastal redwoods.