Theories of the Origin of Multicellular Life There are a number of theories to explain the origin of multicellular life. Most of the theories are derived from studies of various types of cells and living organisms, including advanced protozoans, early developmental stages (embryos), and larval stages. Four types of cells are central to these theories, and they are grouped into two categories: motile (capable of movement) and nonmotile (not capable of movement). The motile protists include flagellate cells (those with a whiplike "tail," or flagellum) and amoeboid cells (those such as Amoeba, which move by pseudopodia, or fingerlike extensions of the cell membrane). The nonmotile stages include coccine cells (those with many nuclei, sometimes called multinucleate cells) and sporine cells (those that divide and stick together to form multicellular aggregates). There are many theories that have been proposed to explain the origin of plants, fungi, and metazoans (animals). Formerly, it was thought that plants evolved from prokaryotic algae (cyanobacteria, or blue-green algae), but it is more likely that plants arose from a eukaryotic ancestor, such as a flagellate cell. Flagellate algae are similar to flagellate protozoans, but it is not certain whether the algae evolved fromthe protozoan or vice versa. The presence of plastids (such as the chloroplasts that contain chlorophyll used for photosynthesis) may be the key feature separating plants from protozoans, fungi, and animals. According to a theory proposed by Lynn Margulis, plastids evolved from prokaryotic blue-green algae that were captured by eukaryotic cells. The sporine cell is another possible ancestor of the plants. Sporine cells appear to have had the capacity to evolve beyond the colony level and to produce complex tissue-level green algae and higher plants. Fungi used to be considered as plants that had lost (or never evolved) chlorophyll. The discovery of a single-celled stage with flagellae among the more primitive fungi, however, suggests that fungi probably evolved from protozoans. The ancestral multicellular organisms, which gave rise to all the more-complex living animals, are all extinct. The simplest multicellular animal living today is the sponge (phylum Porifera). The sponges are not considered to be ancestors of the more complex animals because their body organization and developmental history are very different. Sponges have no tissues, mouth, or internal organs. Instead, they consist of an aggregate of flagellate and amoeboid cells (and a few other types) roughly arranged in layers. The sponges may have evolved independently from the other metazoans. Sponges are classified as a distinct side branch of the animal kingdom (Parazoa), with a primitive multicellular grade of organization (no tissues). The remaining multicellular animals are grouped into the Eumetazoa.
Humans and Hibernation Considerable research related to hibernation and its implications for humans is ongoing. Noting that when animals' bodies approach the freezing point, they feel little pain and bleed little, if at all, when they are cut, anesthesiologists and other physicians have begun to simulate some conditions of hibernation in the operating room. Only in the last third of the twentieth century was it possible to perform some kinds of delicate, lifesaving surgery by packing the body in ice and chilling it to the point that most vital signs are significantly reduced. Open-heart and transplant surgery have advanced considerably by using such techniques. The loss of blood accompanying such surgery in earlier eras has been substantially reduced by operating on chilled bodies, then gradually warming them to normal temperatures. The need for anesthetics also has been greatly reduced, thereby eliminating much of the risk that anesthetization frequently carries. Hibernation among humans would be useful in space travel, where reducing their vital functions for extended periods would curtail the need for oxygen and food among space venturers.
