Multicellular Organisms

A few prokaryotes have adopted a multicellular way of life. Stromatolites, for example, consist of bacterial colonies. In general, though, prokaryotic cells live a solitary life (and even in the case of stromatolites it is debatable whether the term "organism" is warranted.) For most of Earth's history, eukaryotic cells also lived isolated lives. Then a remarkable transformation occurred. Some eukaryotic cells discovered the benefits of joining together. Because the cells had no external walls isolating them from the environment and from each other, they were free to exchange information and to share materials. The result was the world we see today: three kingdoms of organisms that are hugely complex and various — fungi, plants and, most complex of all, animals.

What caused eukaryotic cells to pool their resources is not known. It is not even entirely clear when the switch to multicellularity occurred. A crucial event in the history of life was the Cambrian explosion 540 million years ago, which saw the various animal body plans laid down, and which seems to have been a key step on the path to intelligent life on Earth. The

Cambrian explosion saw the fossilization of a broad assortment of animals — so animals were certainly in existence at that time. There are few, if any, animal fossils in rocks older than 540 million years. However, all we can deduce from this observation is that large animals with hard body-parts became common in the Cambrian period. It is entirely possible that small soft-bodied animals were in existence before the Cambrian period and died leaving no trace. (Nematodes are perhaps the most abundant type of animal in the world today. They must have existed since at least the Cambrian explosion, yet they have left no trace in the fossil record.) Gene sequencing leads some biologists to believe animals originated 1 billion years ago, which, if true, means the fossil record relates to only half of the history of animal life on Earth. Whether animals originated a billion years ago, half a billion years ago or some time in between, the fact remains they are johnny-come-latelies in the history of Earth. Single-celled creatures had been around since soon after the Earth cooled; it took 3 billion years for complex creatures to develop. Why the long wait for multicellularity?

One (still controversial) suggestion is that a rise in the oxygen content of the atmosphere ignited the Cambrian explosion.224 Early in Earth's history there was essentially no free oxygen. This lack of oxygen posed no hardship for primitive prokaryotes; indeed, for the first living organisms, and even for some present-day bacteria, exposure to oxygen meant certain death. However, organisms such as cyanobacteria produced oxygen as a by-product of their metabolism. For 2 billion years — from about 3.7 billion years ago to about 1.7 billion years ago — these organisms pumped oxygen into the environment. For most of that time there were enough sinks, such as iron dissolved in the oceans, to trap the oxygen. Eventually, though, the sinks became full — and the oxygen content of the atmosphere began to rise. For many organisms, this event spelled doom; the "oxygen crisis" must have created the biggest of all mass extinctions, with many prokary-otic species simply failing to adapt to the large-scale release of such poison. Some organisms, though, prospered: they evolved a metabolism based on oxygen, breaking down food into carbon dioxide and water. This oxygen metabolism generated more energy than did the anaerobic metabolisms, and the organisms prospered; the eukaryotes prospered most of all. Even until about 550 million years ago, however, the concentration of oxygen in the atmosphere and dissolved in the oceans was far less than present-day amounts. Any animals existing before this period must have obtained oxygen for their tissues by diffusion, which is a slow process. Those animals would have had no heart — at least, no pump — nor would they have possessed a circulatory system. They would have been small, gossamer-like creatures, so it is small wonder that they left no trace in the fossil record. But then, for some reason that is not entirely clear, the atmospheric oxygen level rose yet again in the Cambrian period. Several key evolutionary developments took place — gills, haemoglobin in blood, hearts — allowing marine animals to make much more efficient use of oxygen and to transport the gas to different tissues. Animals became bigger and bulkier and were able to develop various specialized organs. Perhaps the emergence of a predator caused other species to evolve protection in the form of hard shells — and finally animals could become fossils.

The suggestion, then, is that the Cambrian explosion was caused by a rise in the level of oxygen in the atmosphere. And maybe this was a less-than-inevitable occurrence. Perhaps on most planets the development of large multicellular organisms does not take place.

As we have seen, there were many steps leading from simple unicellular organisms to complex organisms consisting of groups of cells working together. On Earth, it took billions of years for these steps to occur and for animals to appear. Which of these steps were vital and the timescale for these steps to occur are still a matter for debate. And it may be that some of the steps required environmental rather than biological changes.

It is at least a plausible resolution of the Fermi paradox that life elsewhere in the Galaxy has stalled at the unicellular stage. We may one day visit planets and find everywhere oceans teeming with strange, microscopic organisms — lots of life, but life at a low grade. Perhaps nowhere else did the right sequence of biological and environmental events take place that would make possible the evolution of animal life — and thus intelligent species with which we can communicate.

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