S

3,000 3,500

4,000

4,500

FIGURE 1-35 Landmarks in the evolution of life on Earth.

Diversification of multicellular eukaryotes (plants, fungi, animals)

Appearance of red and green algae

Appearance of endosymbionts (mitochondria, plastids)

— Appearance of protists, the first eukaryotes

Appearance of aerobic bacteria Development of O2-rich atmosphere

Appearance of photosynthetic O2-producing cyanobacteria

Appearance of photosynthetic sulfur bacteria Appearance of methanogens

— Formation of oceans and continents

— Formation of Earth

Details of the evolutionary path from prokaryotes to eu-karyotes cannot be deduced from the fossil record alone, but morphological and biochemical comparisons of modern organisms have suggested a sequence of events consistent with the fossil evidence.

Three major changes must have occurred as prokaryotes gave rise to eukaryotes. First, as cells acquired more DNA, the mechanisms required to fold it compactly into discrete complexes with specific proteins and to divide it equally between daughter cells at cell division became more elaborate. For this, specialized proteins were required to stabilize folded DNA and to pull the resulting DNA-protein complexes (chromosomes) apart during cell division. Second, as cells became larger, a system of intracellular membranes developed, including a double membrane surrounding the DNA. This membrane segregated the nuclear process of RNA synthesis on a DNA template from the cytoplasmic process of protein synthesis on ribosomes. Finally, early eukaryotic cells, which were incapable of photosynthesis or aerobic metabolism, enveloped aerobic bacteria or photosynthetic bacteria to form endosymbiotic associations that became permanent (Fig. 1-36). Some aerobic bacteria evolved into the mitochondria of modern eukaryotes, and some photosynthetic cyanobacteria became the plastids, such as the chloroplasts of green algae, the likely ancestors of modern plant cells. Prokary-otic and eukaryotic cells are compared in Table 1-3.

At some later stage of evolution, unicellular organisms found it advantageous to cluster together, thereby acquiring greater motility, efficiency, or reproductive success than their free-living single-celled competitors. Further evolution of such clustered organisms led to permanent associations among individual cells and eventually to specialization within the colony—to cellular differentiation.

The advantages of cellular specialization led to the evolution of ever more complex and highly differentiated organisms, in which some cells carried out the sensory functions, others the digestive, photosynthetic, or reproductive functions, and so forth. Many modern mul-ticellular organisms contain hundreds of different cell types, each specialized for some function that supports the entire organism. Fundamental mechanisms that evolved early have been further refined and embellished through evolution. The same basic structures and mechanisms that underlie the beating motion of cilia in Paramecium and of flagella in Chlamydomonas are employed by the highly differentiated vertebrate sperm cell.

Anaerobic metabolism is inefficient because fuel is not completely oxidized.

Bacterium is engulfed by ancestral eukaryote, and multiplies within it.

Symbiotic system can now carry out aerobic catabolism. Some bacterial genes move to the nucleus, and the bacterial endosymbionts become mitochondria.

Nonphotosynthetic eukaryote

Nonphotosynthetic eukaryote

Anaerobic metabolism is inefficient because fuel is not completely oxidized.

Bacterium is engulfed by ancestral eukaryote, and multiplies within it.

Ancestral anaerobic eukaryote

Aerobic bacterium

Aerobic metabolism is efficient because fuel is oxidized to CO2.

Photosynthetic cyanobacterium

Light energy is used to synthesize biomolecules from CO2 .

Engulfed cyanobacterium becomes an endosymbiont and multiplies; new cell can make ATP using energy from sunlight.

Ancestral anaerobic eukaryote

Chloroplast

Photosynthetic eukaryote

Chloroplast

Aerobic bacterium

Aerobic metabolism is efficient because fuel is oxidized to CO2.

Photosynthetic cyanobacterium

Light energy is used to synthesize biomolecules from CO2 .

Engulfed cyanobacterium becomes an endosymbiont and multiplies; new cell can make ATP using energy from sunlight.

Photosynthetic eukaryote

In time, some cyanobacterial genes move to the nucleus, and endosymbionts become plastids (chloroplasts).

FIGURE 1-36 Evolution of eukaryotes through endosymbiosis. The earliest eukaryote, an anaerobe, acquired endosymbiotic purple bacteria (yellow), which carried with them their capacity for aerobic ca-tabolism and became, over time, mitochondria. When photosynthetic cyanobacteria (green) subsequently became endosymbionts of some aerobic eukaryotes, these cells became the photosynthetic precursors of modern green algae and plants.

TABLE 1-3 Comparison of Prokaryotic and Eukaryotic Cells

Characteristic

Prokaryotic cell

Eukaryotic cell

Size

Generally small (1-10 ^m)

Generally large (5-100 ^m)

Genome

DNA with nonhistone protein;

DNA complexed with histone and nonhistone

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