Planet for lease

Feeling nostalgic? Here's a timeline showing some of the important events on planet Earth since it formed approximately 4.5 billion years ago. My personal favourite would have to be the Cambrian explosion of 500 million years ago (see 'First Hard-shelled Animals'). This was not literally an explosion, but a relatively short time period in which the major groups of modern invertebrates evolved. And I suppose anyone's 'best of' list would also have to include that classic moment, the evolution of life, which happened roughly 3.5 billion years ago.

One thing I find striking about this sort of diagram is the 1 billion years between the formation of our planet and the appearance of anyone on it. For a little less that a quarter of the Earth's history, there was nobody here. I find this concept very calming to think about, particularly while using public transport during peak hour. Thinking about the empty earth is the biologist's equivalent of the Zen Buddhist puzzle about whether a tree falling in a forest still makes a sound if no one is around to hear it. A more scientifically minded version of the question would be: If a bubble of volcanic gas erupted on the primeval Earth, and no biotic molecule had evolved yet to inhale the vapour, would it still smell like methane?
In the period before life formed, there was constant volcanic activity. This produced an atmosphere of hydrogen and ammonia gases, water vapour, methane, carbon monoxide and carbon dioxide. The oxygen rich atmosphere we enjoy today had not yet formed. It took roughly half a billion for the Earth's crust to solidify.

The planet was constantly struck by lightning and bombarded by meteorites. The atmosphere was easily penetrated by ultraviolet radiation, which may have been especially strong because it radiated from a young sun.

In short, Earth before life evolved was characterised by a series of natural disasters. Yet, in the absence of any life forms to suffer the outcome of all this chaos, even the most catastrophic events are reduced to inconsequential sound, colour and movement. Or they would would be, if anyone had been around to hear, see or feel them. For one billion years, there was complete peace on Earth.

Conditions on the young Earth seem extremely hostile to life, but it was probably these very conditions that allowed life to form. In 1953, Stanley Miller and Harold Urey used a large glass flask to recreate this environment. The flask was filled with water, representing the ocean, while the 'atmosphere' was composed of the gases listed above. Electrical sparks were discharged to simulate lightning. The aparatus was left to sit as a self-contained system. This experiment was something very like scientists playing God.

And, amazingly, Miller and Urey managed to replicate God's data. Within a week, the flask contained amino acids. These molecules are the building blocks for proteins, which in turn form most of the structural components of animals. Further lab recreations of the primitive Earth have produced all 20 of the amino acids found in living organisms, as well as sugars, lipids, and the bases for DNA: in short, the building blocks necessary for life.

The atoms needed to make these molecules are present in the atomospheric gases. It is thought that energy provided by the lightning, and perhaps by UV radiation, enabled the breaking and reforming of chemical bonds. Our modern, oxygen rich atmosphere prevents the formation of new bonds, but on a low-oxygen planet, this process would have been possible.

In order for life to form, the tiny molecules had to join together to form larger molecules. In laboratory experiments, organic molecules have been joined together by a process that involves diluting them in water and dripping them onto hot rocks, sand or clay. It is thought that the many organic molecules dissolved in seawater may have been carried by waves onto hot rocks or larva, forming proteins, before being washed back into the ocean.

Also under laboratory conditions, these larger molecules have been shown to form what are known as 'protobionts': aggregations of molecules that are not considered living, but have some of the properties of living things. Protobionts have been observed to form an outer layer of lipids that resembles a cell membrane, and to show signs of primitive metabolism and electrical excitability, the latter of which is necessary for the development of a nervous system. These observations do not prove that life evolved in this way, but they do show that this course of events would have been possible.
On a planet crowded with the protobionts' possible descendants, perhaps the only places where the relative peace of the primeval Earth still exists is in space. But then again, maybe not. Meteorites hitting the Earth in the modern era have been found to carry amino acids. If the idea that we had our origins in the action of waves on hot rocks is difficult to comprehend, this evidence makes the picture even stanger. Some scientists believe that we had at least some of our origins in space, and that our precursor molecules were brought to Earth by meteorites and comets. If amino acids could form on other planets, there is also a possibilty that life has formed on planets other than Earth. If we have company, the universe could be even noiser than we imagined.