Christian Nusslein-Vothard, a Nobel Prize Laurence in Physiology and Medicine, is the
author of Coming to Life: How Genes Drive Development, from which this article was
. Vol. 139, Iss. 2784; pg. 31, 4 pgs
ANIMALS DISPLAY an extreme diversity of form, lifestyle, and body organization.
Their ancestors were built simpler than the species existing today. One of the most
interesting questions in biology is how these forms evolved over time and which
innovations of their body plans helped them adapt to new conditions. The answer is
difficult to come by because, in most cases, neither the intermediate forms nor the
common ancestors exist anymore. Fossils, which are the only clue to the animals of the
past are very rare and tell us almost nothing about the embryonic development or the
genes of these animals.
Animals can be classified based on their similarities and thus their evolutionary
relationships into groups known as taxa. Members of a particular taxon all share a
common ancestor. The largest taxa are called phyla and there are approximately 30 of
them today, though many more probably existed in the past. The five phyla that include
the highest number of animal species are the nematodes, or round worms; annelids, or
ringed worms; mollusks; the arthropods: and chordates. These phyla, in turn, are divided
into classes and each class then is subdivided into orders, families, genera, and species.
For instance, the largest class of the chordates is the vertebrates. The vertebrates
themselves are divided into the five orders of fish, amphibians, reptiles, birds, and
mammals. The mammals include Homo sapiens - the humans.
The relationship among different species can be recognized more easily in embryos than
in adults because, during embryonic development, an animal's basic construction plan
becomes apparent. This body plan is displayed most clearly at a stage when the animal is
not yet fully developed, and when it is not yet able to feed itself. At this early stage, the
body still is comparatively simple, because the structures necessary for adaptation to a
specific lifestyle have not yet developed. Therefore, animals often are grouped as similar
based on embryonic structures rather than on adult ones. The vertebrates, for example,
are subdivided into amniotes (repules, birds, and mammals) and anamniotes (fishes and
amphibians) based on whether they possess an amnion - a protective embryonic covering.
The amnion. however, is not the only criteria for this classification, as there are insects
that form an amnion. The fact that some vertebrates have an amnion is based on
homology, in other words, derivation from a common ancestor, while the presence of an
amnion in vertebrates and insects is based on analogy, namely similar function that has
arisen independently during the evolution of the two phyla.