Quantum Evolution: Life in the Multiverse. Johnjoe McFadden
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Surprisingly perhaps, eight hundred to a thousand genes is a little more than the number of genes present in the genome of a living bacterium, Mycoplasma genitalium. This organism causes non-gonoccocal urethritis (inflammation of the urethra which is not gonorrhoea) and respiratory infections in humans, and has the smallest known genome of any living creature. Its entire chromosome of 580,070 DNA bases has been sequenced and found to code for only four hundred and seventy proteins. However, this microbe is not really a free-living organism – it lacks the enzymes necessary to make many essential biochemicals. It barely manages to replicate under very cosseted laboratory conditions. It is actually a highly evolved parasite that relies on our cells to do much of its biosynthetic hard work. It is not a feasible proto-cell. Nevertheless we will accept a lower estimate of about five hundred genes as the minimum number of genes likely to have been present in the last common ancestor of all cellular life, the proto-cell.
But what was the proto-cell like? We can approach this question by examining the characteristics of the deepest branches of the rRNA tree. For instance, the deepest branching Archaea are thermophilic bacteria that breathe sulfur compounds. The deepest branches of the eubacterial domain are thermophilic (heat-loving), sulfur-utilizing photosynthetic bacteria; suggesting that the proto-eubacterium was an anaerobic photosynthetic cell. Most researchers do not think it likely that photosynthesis evolved very early in life. It is a highly complex reaction that requires the co-operative action of many enzymes. The more ancestral characteristics are likely to be closer to the Archaea that use the energy of sulfur compounds directly to fix carbon dioxide. On this basis, the likely characteristics of the proto-cell were probably similar to the Archaea, suggesting it lived in a hot, sulfurous, oxygen-free environment and it used sulfur to fix carbon dioxide.
But when did the proto-cell live? Do the conditions we have described correspond to the likely environment where it emerged?
THE FIRST FOSSILS
The first undisputed evidence of life is in rocks about three and a half billion years old. Large circular structures called stromatolites (from the Greek meaning ‘stony carpet’) are present in fine-grained flint-like sedimentary rock7 called chert. Some of these rocks contain curious white concentric rings about a metre across. Nobody knew the origin of these structures until in the 1950s scientists cut sections of the rock and examined them under the microscope. They found rods, spheres and chains of cells, unmistakably the fossilized remains of microbes. The oldest stromatolite microfossils were found by the University of California’s paleobiologist J. William Schopf, in chert outcrops near Chinaman Creek in Western Australia dated to three and a half billion years ago. Schopf found many short rods and filaments in the rock very similar in size and shape to the cells of modern bacteria, including the photosynthetic cyanobacteria of Chapter Two. Some of the fossils were incredibly well preserved, allowing Schopf to discern their internal structure and even recognize cells that appeared to have been undergoing cell division before their lineage was abruptly petrified. Cells of various sizes and shapes were present suggesting that a diverse microbial community was already thriving.8
Rocks even more ancient than those of Chinaman Creek are found on Isua in West Greenland. These date back about 3.85 billion years. Nobody has yet found any microfossils in these rocks but this is unsurprising since they have gone through episodes of heating at high pressures that would have destroyed the structure of any ancient organisms. Another way of looking for evidence of past life is to examine the ratio of carbon-12 to carbon-13 isotopes9 in sedimentary rock. Plants and bacteria that convert inorganic carbon (as carbon dioxide, carbonate or methane) to biomass discriminate against carbon-13 so their tissues are enriched by the lighter carbon-12 isotope. Rocks that hold the fossilized remains of ancient carbon-fixing plants and microbes preserve this preponderance. Microfossil-bearing rocks including the ancient cherts show the unmistakable signature of biological carbon fixation. Although the Isua rocks contain no fossils they do have grains of graphite within them which could be the roasted remains of ancient microbes. Scientists from the Scripps Institution of Oceanography, University of California, examined the Isua graphite and found enriched levels of carbon-12, indicating that the carbon may possibly be of biological origin. The levels were not as high as in later rocks, possibly due to high temperature re-equilibration of the isotope ratios, but do suggest a biological origin.
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