Life may change.
Percy Bysshe Shelley, Hellas
For some biologists, the haste with which cells appeared on Earth implies that the generation of life from inanimate matter is straightforward. If Earth is typical, then millions of planets in the Galaxy may be home to microbial life. However, although the eukaryotes might be as old as the archaea and bacteria, the byzantine biochemical machinery of the modern eukaryotic cell took a long time to reach its present level of sophistication. it may have taken a billion years; maybe longer. The development of large mul-ticellular organisms took longer still. This is not necessarily surprising: eukaryotic cells are immensely more complex than prokaryotic cells, and several evolutionary developments had to be made before different eu-karyotic cells could learn to cooperate and function effectively in groups. But perhaps this long time implies that the development of the eukaryotic grade of life follows a tortuous, difficult path. Presumably, complex multi-cellular life anywhere in the Galaxy must evolve from single-celled micro-bial life. Maybe complex eukaryotic life — and thus life capable of communicating over interstellar distances — has not yet developed on other planets. Perhaps this explains the silence of the Universe. Perhaps the Galaxy is filled with planets on which life has stalled at the prokaryotic stage.
What led to the change from the prokaryotic grade of life, which dominated life on Earth for so long, to the eukaryotic grade of life we see all around us today? To answer that — and to attempt to understand whether the eukaryotic grade of life might be a rare phenomenon — we need to understand something of the differences between two types of cell.
Differences Between Prokaryotic and Eukaryotic Cells
Whichever way you consider it, bacteria have always been the most successful life-forms on Earth. Their simplicity, combined with their capacity to reproduce quickly, almost guarantees success. They evolve biochemical responses to environmental challenges, so even though they all tend to look alike, different bacterial species possess different metabolisms and can inhabit a wide variety of niches. They are also extremely hardy, and some species seem to have survived unchanged for billions of years.
Complex eukaryotic life-forms such as plants and animals are much less robust. They are prone to mass extinctions, and even in the natural run of things the typical lifespan of an animal species is measured in millions rather than billions of years. Nevertheless, the eukaryotic grade of life is much more interesting than the prokaryotic grade. Eukaryotes evolve morphological responses to environmental challenges — in other words, they develop new body shapes and body parts — which leads to a variety and freshness absent in the prokaryotes.
A major difference between eukaryotic and prokaryotic cells is that the latter have rigid cell walls or very rigid cell membranes, whereas eukary-otic cells either lack cell walls or have very flexible walls. This flexibility allows eukaryotic cells to change shape, and also to engage in cytosis — a process wherein the cell membrane pushes inward to form an intracellular vacuole. Many cellular processes employ cytosis, but perhaps its main role is in phagocytosis. In phagocytosis, a eukaryotic cell engulfs a particle of food into a food vacuole, where enzymes then digest it. Obtaining nourishment like this by predation is a much more efficient process than that employed by bacteria, which secrete digestive enzymes into the surrounding medium and then absorb the resulting molecules.
Another distinguishing characteristic is that a eukaryotic cell has a nucleus, separated from the cytoplasm by two membranes, which contains the cell's DNA. Eukaryotic cells also contain organelles — little organs — which are separated from the cytoplasm by membranes. The organelles include the mitochondria (which play a vital role in energy metabolism) and the plastids (which play a role in photosynthesis in plants and algae). In the early 1970s, Lynn Margulis argued that organelles must have arisen by symbiosis. She reasoned that, billions of years ago, very primitive eukary-otic cells would have used phagocytosis to ingest smaller prokaryotic cells for food. Some prokaryotic cells might have been indigestible and would have stayed in the larger eukaryotic cells for some time. And some of those prokaryotes would have performed functions — such as the transformation of energy — more efficiently than their hosts. Both cells would benefit from partnership — and both would have a selective advantage when it came to passing on their genes. An initially indigestible bit of food would become indispensable to the smooth running of a eukaryotic cell. Support for Margulis' idea has come from DNA sequencing. Mitochondria and plastids have their own DNA, which is different from the DNA in a cell's nucleus. It turns out that mitochondrial DNA and plastid DNA are much closer to prokaryotic than eukaryotic DNA. The mitochondria, for example, probably share a closest common ancestor with present-day symbiotic purple non-sulfur bacteria. (Direct evidence for Margulis' hypothesis has probably been erased by a billion years of evolution, but the hypothesis makes so much sense that it is widely accepted.)
Another major difference exists between the two cell types. Unlike prokaryotes, new eukaryotes can form through the fusion of gametes from two parents; in other words, sex can occur. Furthermore, the amount of genetic information stored by eukaryotes (and passed on either through sex or through parthogenesis) is far greater than that stored by prokaryotes.
Finally, eukaryotes possess a cytoskeleton. The cytoskeleton consists of actin filaments, which resist any pulling forces that might act on a cell, and microtubules, which resist any shearing or compression forces that might act on a cell. Thus, even in the absence of a rigid cell wall, a eukaryotic cell can maintain its shape and integrity. But the cytoskeleton can do much more: it can draw the cell into a variety of temporary shapes, it marshals the organelles into various positions, and it allows the eukaryotic cell to increase in size. Actin and tubulin — the structural proteins from which the cytoskeleton forms — are thus among the most important of all proteins for the development of complex life.
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