3. The Evolution of Cells

3.4 The Appearance of the Metazoa

It is highly unlikely that large amounts of free oxygen in the atmosphere of the earth could have been produced before the advent of oxygen-producing blue-green algae. The only significant abiotic source of atmospheric oxygen is provided by the decomposition of water vapor into hydrogen and oxygen in the upper atmosphere by sunrays, followed by the escape of hydrogen to space. However, few of the freed hydrogen atoms are blown off. Most of them recombine to form water, leaving no free oxygen.

Furthermore, the primitive earth was warmer than today and volcanic sources of hydrogen were great enough to significantly deplete the atmosphere of all the free oxygen it contained.

The advent of photosynthesis about 2.5 billion years ago introduced a new source of atmospheric oxygen, which presumably increased with time while the volcanic hydrogen source decreased because the interior of the earth cooled down. Eventually came a time when the rate of release of oxygen into the atmosphere exceeded the rate of release of the hydrogen. This transition from a reducing atmosphere to an oxidizing one could have been quite rapid on the geological time scale and has been dated around 2.5 billion years ago.

The variable response of bacteria to ambient oxygen is consistent with the view that they evolved in response to a rising oxygen tension. By contrast, eukaryotes are all aerobes. There is no eukaryotic animal or plant that completes its entire life cycle in the total absence of oxygen. The few exceptions, such as yeasts or unicellular eukaryotic flagellate parasites of the intestines of termites, seem to derive from aerobic ancestors and require metabolites such as unsaturated fatty acids and sterols, made by oxygen-utilizing pathways, sometimes provided by the termite hosts or present in the diets of these hosts.

The conclusion to be drawn is that the appearance of eukaryotes was dependent on the presence of an oxidizing atmosphere but there is no evidence that the mere appearance of molecular atmospheric oxygen was sufficient to provide for the appearance of multicellular eukaryotic organisms. Several hundred million years elapsed between the appearance of both an oxidizing atmosphere and eukaryotic unicellular organisms on one hand and that of abundant Metazoan fossils at the end of the Precambrian era on the other hand.

The late appearance of the metazoa cannot either be convincingly explained by a selective pressure of ultraviolet light because ozone appeared well before that and also because protection against lethal UV light was devised by prokaryotes and thus apparently relatively easily obtainable. An unprotected microorganism exposed to full sunlight would be killed in a matter of seconds. Ethane, ammonia, water vapor, nitrogen or carbon cannot provide an ultraviolet screen. Pure water absorbs only weakly in the ultraviolet. In an ozone-free atmosphere, at least a meter of water is needed to protect unshielded organisms. Besides growth at a safe depth of water, prokaryotic organisms have devised a number of biological mechanisms to protect themselves from UV damage. These consist in nucleic acid repair systems, of growth among solutions of nitrites and nitrates that protect against UV light damage, by growth in mats (stromatolites). Yet, these protective mechanisms devised by prokaryotic cells had little effect on the later origin and evolution of eukaryotic cells.

Ozone forms extremely rapidly when oxygen is present and substantial densities of ozone can exist at very low partial pressures of oxygen. The ozone screen against UV light was established before the abundance of atmospheric oxygen reached 1% of its present level. Ultraviolet light may very well have disappeared from the surface of the earth long before the end of the Precambrian era and its absence cannot be a factor for the sudden appearance of the metazoa at later times. Some bacteria are provided by a mechanism of ozone destruction, indicating that the evolution of some bacterial species took place as a response to the rising partial pressure of this gas. Eukaryotes and a fortiori metazoans would have evolved later, when the gas was already present.

A problem that is rarely raised 15 is the tolerance of early organisms to salt. The salinity of the early ocean was about twice the modern value. There was some continental crust about 3.5 billion years ago and another continental mass developed at about 2.5 billion years. Two further masses appeared at about 2 billion years, from which time assembly and break-up of continents dominate the history. Before the continental crust developed, all the salt and brine now found on the continents would have been entirely in the oceans. Large scale deposition of salt within giant sedimentary basins on continents could not have begun before 2.5 billion years and the decrease in salinity could have persisted until the time of the Cambrian ‘explosion’ of life at 0.5 billion years. Cyanobacteria are more salt tolerant than most organisms and these dominate the Precambrian fossil record. Oxygen solubility in seawater decreases significantly as salinity (and temperature) increase. The rise in oxygen level would have been retarded in a more saline ocean. Further, most forms of modern multicellular life cannot tolerate salinities above 50 per thousand.

The considerable length of time that elapsed between the appearance of the first eukaryotic cells and that of the first Metazoans is to be drawn, at least in part, on a simple phenomenon of biological divergence based on the development of the sexual system of transfer of information. We may assume that the sexual system arose sometime between 2 and 0.9 billion years ago, after the transition to the oxygenic atmosphere. This sexual system is extremely complex and required a considerable length of time to evolve in a manner adequate to sustain further evolutive trends. Very probably, the absence of the synthetic apparatus needed to synthesize stabilizers of nucleic acid, namely putrescine, spermidine and spermine, was the reason for this delay. Spermine is found only in evolved eukaryotic cells.

Figure 3.19: Putrescine is a small molecule ending up with two amino groups (NH2). Next in complexity is spermidine, which is much longer than putrescine. Spermine is the end product of this series of basic molecules. These basic polyamines aggregate with nucleic acids and protect them.

An analysis of the sexual system used among protists shows that considerable variations occur in the mitotic systems of amoebae, dinoflagellates, flagellate algae, radiolarians, ciliates, etc. and suggests that stabilization of the system was a prerequisite for the subsequent appearance of metazoans. Apparently those protists which succeeded in removing calcium and depositing it as an external shield around their body enjoyed an intracellular stabilization of the mitotic apparatus further maintained by spermine, while in the meantime they became preadapted for the formation of protective and supportive calcium carbonate robust parts. Calcium carbonate shell deposition could have originated in populations of protists that regulated intracellular calcium ion concentrations in carbonate-rich waters. The primitive group of coelenterates, from which stem the totality of the subsequently evolved forms of animal life, are for their main part relying on an external shield of calcium carbonate (corals) for their protection and support. As the free form of jellyfishes, they are unprotected. These animal forms are constituted mainly of a kind of gelatin. Gelatin can take up as much as 25 times more oxygen than does water. With this capacity to concentrate and store oxygen from the ambient medium, the coelenterates were the choice material for the next evolutive step.

The emergence of metazoans occurred more than 12 million years before the base of the Cambrian. The oldest known bilateral fossil, Kimberella at 555 million years, is found in South Australia and the White Sea. It does not allow the conclusion that the split protostome-deuterostome had occurred at that time. Trace fossils indicate that metazoans capable of active crawling and burrowing were present by that time.

References

15. L. Knauth: salinity history of the earth’s early ocean. Nature 395, 554, 1998

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