p265

複雑系はいかに複雑化してきたか．

神経系の進化に関して典型的に的を外した解説がScienceに出ている．それを批判したコメントをブロッグに書いたが，それを転載する．神経系の発展には本書で説明した複雑化のシナリオがかなり素直に成り立っていると考えられる．ところが，この解説は足を踏み外せるところはことごとく踏み外しているようにさえ見える解説である．事実のまとめはありがたい．それは著者の主張を裏付けていると見える．

Greg Miller

On the Origin of The Nervous System   

Science 325 24 (2009)

以下に事実に関した部分を多少整理して引用しておく

* Amphimedon queenslandica reported in PLoS ONE in 2007 contain a set of proteins typically found on the receiving side of a synapse, although electron microscope studies have failed to find synapses in sponges. Although their genomes contain genes for some neurotransmitter receptors, sponges appear to lack the type of receptors used for most excitatory neural communication in other animals. Cells in the sponge's larvae express a handful of genes that spur neural precursor cells to develop into full-fledged neurons in more complex animals. Inserting the sponge version of one of these genes into frog embryos and fruit fly larvae led to the birth of extra neurons. These cells sit on the outer surface of the sponge larvae.

*Physiological experiments with sponges have also turned up signs of neural foreshadowing. Glass spongeRhabdocalyptus dawsonigenerate action potentials (Leys and Mackie N 1997)

*Sponges have many of the right components, but some assembly is still required.

*However, Ctenophores may be the lowest. If ctenophores came before sponges, the assorted nervous system components that have turned up in sponges may not be foreshadowing after all but rather the remnants of a nervous system that was lost after the sponge lineage split off from that of ctenophores.

*Some jellyfish, such as the bell-shapedAglantha digitale, are more organized, with clearly defined bundles of nerves running around the base of the bell. Cnidarian neurons generate action potentials and release neurotransmitters to communicate across synapses. Nv reveals a surprisingly large array of genes encoding enzymes that synthesize or break down neurotransmitters, as well as receptors for these signaling molecules (Science, 6 July 2007, p. 86).

というわけで，本書に書いたように，複雑化するためにはそのために必要な素材がすでになくてはならないのであり，統合する前に底生生活にはいってしまうと，それらが失われて次の段階にいけなくなるのである．

ここに引用した解説記事では従来の間違ったアイデアが見事に現れている．

Many scientists think Porifera are the living creatures most similar to the common ancestor of all animals. もちろん完全に間違い．

And to many researchers, sponges look like animals on the verge of a nervous breakthrough. もちろんこれは零落した姿に過ぎない．

ChoanoflagellataもMetazoa 進化への道筋を示すものと考えるのはまったく的を外していて，不活発な生活のゆえの退化した姿と見るべきで単純に原始的と見てはいけない．半索動物に関してこの解説記事の著者は ‘’次のように書く:

If the ancestor had a centralized nervous system, several lineages, including that of Saccoglossus , must have later reverted to a diffuse nervous systems an apparent downgrade that is ``hard to explain.’’

もちろん濾過食生活で退化したに過ぎない．