The Emergence of Everything by Morowitz Harold J.;

Author:Morowitz, Harold J.;
Language: eng
Format: epub
Publisher: Oxford University Press USA - OSO
Published: 2014-03-09T16:00:00+00:00

Chapter 14—Readings

Buss, Leo, 1992, The Evolution of Individuality, Princeton University Press.

Margulis, L., and Schwartz, K. V., 1988, Five Kingdoms, W. H. Freeman Co.

Valentine, J. W., Collins, A. G., and Meyer, C. P., 1994, “Morphological complexity increase in metazoans,” Paleobiology, 20:131-142.

15

The Neuron

The key to multicellularity is that a single fertilized egg formed by the fusion of two haploid gametes can give rise in the morphogenetic process to a variety of specialized cells. Thus, switches must be available to turn on and off genetic sequences to distinguish the cell morphology and biochemistry in different tissues. A hair cell is vastly different from a liver cell, which is vastly different from a nerve cell, yet they are all in the clonal progeny of a single fertilized ovum. The number of cell types appears to increase along the evolutionary pathway indicated in the previous chapter.

The descendants of the earliest animals are probably the placazoa and the porifera (sponges). Communication between cells in the protists and early animals is largely chemical. There are two methods: (1) A cell releases into the environment molecules that freely diffuse and may adsorb on the surface of a second cell or be transported across the second membrane to a receptor site; (2) gap junctions may exist between neighboring cells that permit intercellular transport of matter, including signaling molecules. Thus, signaling between cells is limited by diffusion, which is a slow process over large distances, as well as ultimately being concentration limited by dilution.

A nerve cell, on the other hand, receives a chemical signal at a given locus on the surface and converts it into an electrical signal, the action potential. In the electrical form, the signal moves rapidly along the axon and triggers chemical release at contacts with receptor sites of other cells. The axon may be thousands of cell diameters in length, so that a cell-to-cell signal may be sent rapidly over large distances.

Now, animals may be sessile, such as sponges, or very small, such as

dicyemids, and not require nerve cells, but a large and responsive animal will require rapid signalling between remote parts. The emergence of the neuron as a cell type was a critical factor in animal evolution. It may have taken up to a billion years from the first multicellular animal to animals with nervous systems. The agents were cells, and the selection was for cells that could perform certain tasks.

The most primitive nervous system is in the Cnidaria or coelenterates, animals such as hydra or jellyfish. Their axons lack the myelin sheath found in other animals, and the nerves are capable of signal transmission in both directions. The nerve cells form a loose network throughout the organisms. The related phylum Ctenophora, or comb jellies, also have diffusion. For a number of reasons, it appears that nerve cells were derived from epithelial cells. In present-day animal embryos, epithelium and nerves derive from the same tissue layer, indicating a close relationship. The necessity for excitable cell membranes seems clear and can be traced back to protozoa, yeast, and bacteria.

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