Appearance of the Eukaryotes

Appearance of the Eukaryotes
The eukaryotes (“true nucleus”; Figure 2-15) have cells with membranebound nuclei containing chromosomes composed of chromatin. Constituents of eukaryotic chromatin include proteins called histones and RNA, in addition to the DNA. Some nonhistone proteins are found associated with both prokaryotic DNA and eukaryotic chromosomes. Eukaryotes are generally larger than prokaryotes and contain much more DNA. Cellular division usually is by some form of mitosis. Within their cells are numerous membranous organelles, including mitochondria, in which the enzymes for oxidative metabolism are packaged. Eukaryotes include animals, fungi, plants, and numerous singlecelled forms formerly known as “protozoans” or “protists.” Fossil evidence suggests that single-celled eukaryotes arose at least 1.5 million years ago (Figure 2-16).

Molecular sequencing has emerged as a very successful approach to unraveling the genealogies of ancient forms of life.The sequences of nucleotides in the DNA of an organism’s genes are a record of evolutionary relationship, because every gene that exists today is an evolved copy of a gene that existed millions, even billions, of years ago. Genes become altered by mutations through the course of time, but vestiges of the original gene usually persist.With modern techniques, one can determine the sequence of nucleotides in an entire molecule of DNA or in short segments of the molecule.When corresponding genes are compared between two different organisms, the extent to which the genes differ can be correlated with the time elapsed since the two organisms diverged from a common ancestor. Similar comparisons can be made with RNA and proteins.

Because the organizational complexity of the eukaryotes is much greater than that of the prokaryotes, it is difficult to visualize how a eukaryote could have arisen from any known prokaryote. The American biologist Lynn Margulis and others have proposed that eukaryotes did not in fact arise from any single prokaryote but were derived from a symbiosis (“life together”) of two or more types of bacteria. Mitochondria and plastids, for example, each contain their own complement of DNA (apart from the nucleus of the cell), which has some prokaryotic characteristics.

Nuclei, plastids, and mitochondria each contain genes encoding ribosomal RNA. Comparisons of the sequence of bases of these genes show that the nuclear, plastid, and mitochondrial DNAs represent distinct evolutionary lineages. Plastid and mitochondrial DNAs are closer in their evolutionary history to bacterial DNAs than to the eukaryotic nuclear DNA. Plastids are closest evolutionarily to cyanobacteria, and mitochondria are closest to another group of bacteria (purple bacteria), consistent with the symbiotic hypothesis of eukaryotic origins. Mitochondria contain the enzymes of oxidative metabolism, and plastids (a plastid with chlorophyll is a chloroplast) conduct photosynthesis. It is easy to see how a host cell that was able to accommodate such guests in its cytoplasm would have had enormous evolutionary success.

In addition to maintaining that mitochondria and plastids originated as bacterial symbionts, Lynn Margulis argues that eukaryote flagella, cilia (locomotory structures), and even the spindle of mitosis came from a kind of bacterium like a spirochete. Indeed, she suggests that this association (the spirochete with its new host cell) made the evolution of mitosis possible. Margulis’s evidence that the organelles are former partners of the ancestral cell is now accepted by most biologists.
Eukaryotes may have originated more than once. The first eukaryotes were undoubtedly unicellular, and many were photosynthetic autotrophs. Some of these forms lost their photosynthetic ability and became heterotrophs, feeding on the autotrophs and the prokaryotes. As the cyanobacteria were cropped, their dense filamentous mats began to thin, providing space for other organisms. Carnivores appeared and fed on herbivores. Soon a balanced ecosystem of carnivores, herbivores, and primary producers appeared. By freeing space, cropping herbivores encouraged a greater diversity of producers, which in turn promoted the evolution of new and more specialized croppers. An ecological pyramid developed with carnivores at the top of the food chain.

The burst of evolutionary activity that followed at the end of the Precambrian period and beginning of the Cambrian period was unprecedented. Some investigators hypothesize that the explanation for the “Cambrian explosion” lies in the accumulation of oxygen in the atmosphere to a critical threshold level. Larger, multicellular animals required the increased efficiency of oxidative metabolism; these pathways could not be supported under conditions of limiting oxygen concentration.