What does the dreaded "E" word mean, anyway

Natural History, Feb, 2000 by Stephen Jay Gould

In the October 21, 1999, issue of Nature, M. Kobayashi, H. Furuya, and P. W. H. Holland present an elegant solution to this old problem ("Dicyemids Are Higher Animals"). These researchers located a Hox gene--a member of a distinctive subset known only from metazoans and operating in the differentiation of body structures along the antero-posterior (front to back) axis--in Dicyema orientale. These particular Hox genes occur only in triploblastic, or "higher," metazoans with body cavities and three cell layers, and not in any of the groups (such as the Porifera, or sponges, and the Cnidaria, or corals and their relatives) traditionally placed "below" triploblasts. Thus, the dicyemids are descended from "higher," triploblastic animals and have become maximally simplified in anatomy by adaptation to their parasitic lifestyle. They do not represent primitive vestiges of an early stage in the linear progress of life.

In short, if the traditionally "highest" of all triploblasts--the vertebrate line, including our exalted selves--appears in the fossil record at the same time as all other triploblastic phyla in the Cambrian explosion, and if the most anatomically simplified of all parasites can evolve (as an adaptation to local ecology) from a free-living lineage within the "higher," triploblastic phyla, then the biological, variational, and Darwinian meaning of "evolution" as unpredictable and nondirectional gains powerful support from two cases that, in a former and now disproven interpretation, once bolstered an opposite set of transformational prejudices.

As a final thought to contrast the predictable unfolding of stellar evolution with the contingent nondirectionality of biological evolution, I should note that Darwin's closing line about "this planet ... cycling on according to the fixed law of gravity;' while adequate for now, cannot hold for all time. Stellar evolution will, one day, enjoin a predictable end, at least to life on Earth. Quoting one more time from Britannica:

   The Sun is destined to perish as a white dwarf. But before that happens, it
   will evolve into a red giant, engulfing Mercury and Venus in the process.
   At the same time, it will blow away the earth's atmosphere and boil its
   oceans, making the planet uninhabitable.

The same predictability also allows us to specify the timing of this catastrophe--about 5 billion years from now! A tolerably distant future, to be sure, but consider the issue another way, in comparison with the very different style of change known as biological evolution. Earth originated about 4.6 billion years ago. Thus, half of our planet's potential history unfolded before contingent biological evolution produced even a single species with consciousness sufficient to muse over such matters. Moreover, this single lineage arose within a marginal group of mammals--the primates, which include about 200 of the 4,000 or so mammalian species. By contrast, the world holds at least half a million species of beetles. If a meandering process consumed half of all available time to build such an adaptation even once, then mentality at a human level certainly doesn't seem to rank among the "sure bets," or even the mild probabilities, of history.