Life on a Young Planet by Andrew H. Knoll Read Free Book Online Page A

as a reflection of microbial genealogy. But we must think of it as a genealogy of the microbial “chassis,” with specific features that “soup up” well-adapted organisms assembled, in part, by gene exchange across taxa.
    The bacterial limb of the tree is profusely branched; at present we know of at least thirty major groups of bacteria, each more or less equivalent to the plant and animal kingdoms traditionally recognized by biologists. Most of the diverse metabolisms discussed in the previous section can be found on this limb. In particular, photosynthesis is a distinctly bacterial physiology. (How this squares with the obvious presence of photosynthesis in eukaryotic plants and algae is one of the great stories of evolutionary biology, but a tale best left for chapter 8 .) Note, however, that photosynthetic lineages adorn only the upper branches of the bacterial limb. This suggests that Earth’s earliest ecosystems were fundamentally different from those that surround us today. Today, photosynthesis fuels biologyin most habitats. Early life, however, must have run on chemosynthesis. The earliest branches currently recognized on the bacterial limb contain chemosynthetic and heterotrophic organisms, many of which live at high temperatures in the absence or near absence of oxygen.
    In contrast to Bacteria, the Archaea contain only two principal groups, albeit with hints of others yet to be characterized. One branch of archaeans is dominated by methanogenic organisms. Most bugs on this branch are obligate methane producers, but in at least three instances, individual lineages have evolved a more diverse metabolic repertoire that includes respiration. According to Gary Olsen of the University of Illinois, these “add-ons” are all encoded by genes transferred horizontally from bacteria. This emphasizes that horizontal transfer isn’t something that occurred once or twice in the earliest history of life; it is a continuing and persistent means of creating biological novelty.
    Closely related to the methanogenic archaeans are the halobacteria, a distinctive group of microorganisms that gain energy from the sun, using a light-harvesting pigment strikingly similar to the rhodopsin in vertebrate eyes; halobacteria obtain the carbon they need to grow by absorbing organic molecules. The other major branch of the Archaea includes organisms that derive energy from chemical reactions between hydrogen and sulfur compounds.
    Archaea are widely distributed across the Earth, but we still know relatively little about most of them. For example, only in 2001 was it discovered that tiny archaeans may be the most abundant organisms in many parts of the ocean; biologists have no idea how these microbes make their living. On the other hand, some of the best-characterized Archaea live in unusual places— very unusual places. The halobacteria, for example, thrive in waters that are ten times saltier than the ocean. (The striking magenta sheen of halobacteria can be seen from the air in commercial salt ponds, such as those that line the landing approach to San Francisco airport.) Other archaeans live in acid mine waste with a pH of 1. And the current world record holder for temperature tolerance is Pyrolobus fumarii , an archaean that can grow in deep-sea hydrothermal vents at 113ºC. (At the high pressures of the ocean bottom, water this hot remains liquid.) These hyperthermophilic organisms cannot grow at the temperatures used to pasteurize milk—not because they are too hot, but because they are too cold!
    How do we think about such “extremophiles”? Are they merely fascinating oddballs, or do they tell us something fundamental about the history of life? Some, such as the halophiles, reside on distal branches of the Tree of Life, implying that these groups evolved relatively late in its history. In contrast, hyperthermophilic prokaryotes occupy a privileged position in the tree—they are found on the earliest branches of both the