Sidneyia, 22nd of April 2011

fossil, Sidneyia, Burgess Shale, Cambrian explosion

A book "Wonderful Life" (W.W. Norton & Co., New York, 1989) by >> Stephen Jay Gould deals with large number of issues. The basic idea that Gould tries to push is that the evolution is a fairly unpredictable process and that its outcome, i.e. survival of certain species, of certain anatomical plans is not a consequence of hard laws but of an array of coincidences. Such coincidences, but also catastrophes, (often only mildly) prefer a certain type of life, and in a cascade of consequences of such an advantage, a certain history takes place. Such an evolutionary history might have been also a completely different one with only a small change in the randomness i.e. coincidence. We physicists would call that a change in "initial conditions".

There is a concept in a theory of dynamical systems called orbital instability. It means that the trajectories (histories) of a system are such that a small change in initial conditions leads to exponential divergence of the system futures (trajectories) , i.e. that two infinitely similar (but not completely equal) points in history can lead to completely different futures after a long enough time (evolution). Such systems are typically called "chaotic", and the "instability and non-inevitability of the present" Gould calls contingency. The basic experiment that he has in mind is a "repetition" of the flow of evolution with just small changes of (initial) conditions and from that he constructs possible presents / futures that were, in physical terms, almost as probable as the one that we live. The reason that they did not realize cannot be found in physics (e.g. due to better anatomical adjustment or superiority of a particular body design with respect to hydrodynamics) but only in coincidence - only a small coincidence / randomness and we will find Earth in present time without mammals, without terrestrial life except for insects, or even without multicellular life at all, not to mention self-consciousness and intelligence. (I first read about the "repeated" / "replayed history" concept in the book How Nature Works by >> Per Bak, and he did attribute this idea to Gould - but Bak's book is also quite inspiring since he reports on experiments regarding actual rewinding and replaying the history of simple systems.)

Nothing in the creation of Homo Sapiens is inevitable or written explicitly in the evolutionary history - the fact that we are here is a mad luck and irreproducible coincidence of history.

But, that, according to Gould, concerns only a concrete evolution of multicellular life. He still thinks that in the becoming of some life in the conditions on Earth 4 billion years ago there must be some inevitability, some chemical/physical guidance, directionality. That is what I also think.

fossil, Sidneyia, side view

Gould exposes and defends his propositions through a study of (pre)Cambrian fossils discovered in the place in Canada called >> Burgess Shale. His approach is both scientific and historic-scientific, and sociological-scientific (when he discusses the persona and work of >> Charles Doolittle Walcott (1850-1927), a very influential and quite conservative scientist who discovered the Burgess Shale). Gould had a lot of talent for simple exposition of fairly complex issues.

The fossils found in Burges Shale locality are special in many respects, most of all because a lot of them cannot apparently be fitted within the existing taxonomy. Perhaps something in that respect changed since the book was published - I read >> in Wikipedia that the book invited all sorts of reactions, some of them quite negative. However, fundamental Gould's ideas do survive no matter the "correct" interpretation of Burgess Shale fossils, because they are quite general.

In addition to the interesting and many-sided material that Gould produced, the book is notable for its excellent illustrations, among which those made by Marianne Collins stand out. Her 3D fossil reconstructions were made in a technique of precision drawing, and the effects of shadows were produced by "dotting", i.e. by a carefully planned distribution of dots that produce an illusion of shadow and three-dimensional nature of the object. That is a quite laborious technique, but the results that she got are great. That is what gave me the impetus to try to make, on the basis of her reconstructions but also photographs and drawing of the fossils, a three-dimensional model of one of those Cambrian creatures. I opted for Sidneyia, because it seems as if that fossil is particularly well preserved and documented. My reconstructions of this creature follow the text in this post.

fossil, Sidneyia, top view

There are several interesting passages of text that follow and that I wrote down while reading the book:

In the traditional Darwinian view, morphologies have histories that constrain their future, but genetic material does not "age". Differences in rates and patterns of change are responses of an unchanging material substrate (genes and their actions) to variations in environment that reset the pressures of natural selection. But perhaps genetic systems do "age" in the sense of becoming "less forgiving of major restructuring" (to cite a phrase from J.W. Valentine, who has thought long and deeply about this problem). Perhaps modern organisms could not spawn a rapid array of fundamentally new designs, no matter what the ecological opportunity.
fossil, Sidneyia, front view
The basic principle of arthropod design is metamerism, the construction of the body from an extended series of repeated segments. The key to arthropod diversification lies in recognizing that an initial form composed of numerous nearly identical segments can evolve by reduction and fusion of segments, and by specialization of initially similar parts on different segments, into the vast array of divergent anatomies seen in advanced arthropods.
fossil, Sidneyia, back view
fossil, Sidneyia, bottom view

An idea of "modular design" appears in the book several times and that drew my attention because I also thought a lot about that in architectural context, but also in the context of "minimal form", i.e. the form that obeys certain physical constraints by producing equal modules in a certain structure, geometrical relation. For example, the tree solves the problem of maximizing a surface exposed to the light by arranging its "modules"-leaves. Biological solutions are perhaps not the best ones (in the physical sense of minimality i.e. optimality), but they are some solutions. If a recipe for the production of a leaf evolved, then perhaps, by many repetitions of the same recipe one can achieve certain advantage? I intend to return to those ideas sometime.

fossil, Sidneyia, sea

By the way, Kurt Vonnegut (!) is also mentioned in the book and his Galapagos which was inspired by the theory of evolution (see >> HERE what I had to say about it). Gould liked Galapagos more than I did. Gould also mentions Ernst Haeckel, and I also wrote about him a bit >> HERE. Gould writes about him in a somewhat negative context because he holds that Haeckel's illustrations of the "tree of life", i.e. of evolutionary branching, rooted the idea that evolution continually produces more forms, all of them better adapted to the surrounding. On the basis of insight in the Burgess Shale fossils, Gould develops a concept of evolution producing huge number of designs, anatomical plans in a very short time, out of which by "decimation" i.e. dying out in great percentage, only a few survive (not necessarily the "best" ones). Those designs become fixed in evolutionary future and they do not change to some significant extent. That is according to Gould the reason for such differences in design in the present forms of life. In some distant evolutionary past (Precambrian) perhaps there was a continuous transition between different anatomical solutions, designs.

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Last updated on 22nd of April 2011.