Four biological innovations may have been especially significant in paving the way for the emergence of larger animals: (1) the development of sexual cycles; (2) new methods of shuffling information coded along the chromosomes (through the nascent ability to excise and relocate entire gene sequences); (3) new methods of communicating between cells via substances called protein kinases, and (4) the development of a new type of intracellular skeleton, called a cytoskeleton, that allowed eukaryotic cells to increase enormously in size. These innovations greatly enhanced the ability of cells to evolve new morphologies in response to natural selection and their ability to band together into multicellular creatures.
We can now better categorize what we term "advanced" life: eukaryotic multicellular organisms. There are, of course, many types of multicellular organisms, including a considerable number of prokaryotic forms. In most cases these multicellular prokaryotes are composed of only two cell types. Cellular slime molds are multicellular, as are some cyanobacteria. In a way, however, these forms are evolutionary dead ends. They have existed on Earth for several billion years and are highly conservative in an evolutionary sense. It is multicellular creatures of the other category that became so important in the history of life. We refer here to true metazoans.
The jump from single-celled organisms to organisms of multiple cells requires numerous evolutionary steps. The jump from single-celled organisms to metazoan animals, where a high degree of intercellular cooperation in organization exists, involves even more. In their recent book Cells, Embryos and Evolution, biologists John Gerhart and Marc Kirschner discuss this evolutionary accomplishment. The first step, they argue, seems almost paradoxical: It was not some new structure gained that allowed this transition, but an important structure lost. Long ago in our planet's past, some organism of the eukaryotic lineage made a brave (or lucky) morphological change—it shed its external cell wall. Why this occurred is still unclear, but the net effect was far-reaching. A tough outer coating protects most unicellular creatures from their surrounding environment. At the same time, however, it isolates these cells from other members of their own kind. By divesting themselves of this outer wall, individual cells could begin exchanging living material—and information—with one another. The naked cells could adhere to each other, crawl over each other, and communicate. These were the first steps in the formation of a tissue, which is an aggregation of cells united for mutual benefit.
Larger animals require highly integrated systems of cells that can accomplish the myriad functions necessary for all life. Respiration, feeding, reproduction, the elimination of waste material, information reception, locomotion—all require the integration of many cells acting in concert. Each of these functions ultimately requires one or many types of tissues.
Among tissue types, the outer wall of any organism (the epithelium) is of utmost importance. The epithelium must protect the organism from the rigors of the external environment but at the same time allow adsorption of critical gas and, sometimes, nutrients. The evolution of the epithelium was a decisive first step in the evolution of metazoans.
Which group of unicellular creatures first achieved this breakthrough? The most primitive and enigmatic of larger eukaryotic metazoans are sponges. These curious creatures seem to bridge the gap between single-celled eukaryotes or even colonial protozoa and the highly integrated invertebrate metazoan phyla. Sponges have several cell types that perform specialized tasks, but there is a very low level of organism-wide organization. There is no gut or body cavity specialized for processing food, nor is there any nervous system. Yet the sponges may be an important clue to the identity of our actual metazoan ancestors.
The stem, or ancestral, metazoan probably had a larger number of cell types than sponges (perhaps 10 to 15 rather than the 3 to 5 individual cell types found in sponges). There was probably a body cavity of some sort segregated into two cell layers: an outer ectoderm and an inward-facing endo-derm. This two-tissue plan seems to have been an evolutionary dead end, and it wasn't until a third layer—the mesoderm—was added that animals with real internal complexity formed. Eventually, a small worm-like shape with three tissue layers evolved, a creature with a gut running through the long axis of the body and a separate space known as a coelom to serve as an internal hydrostatic skeleton. With this tiny organism (the first may have been less than a millimeter long), the evolutionary stage was set for the emergence of animals on planet Earth.
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