The search for life: Astrobiology

By Monica Grady

The evolution of life

In this session we will explore the evolution of life, as this has to be the basis for any attempt to study the origin of extra-terrestrial life. The ingredients for life include carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur, with a substrate of water, and energy from both the sun and chemical reactions. Carbon is by far the most important element, forming the basis of life on Earth. The fossil record stretches back to between 3.5 and 3.85 billion years ago, but it took until the start of the Cambrian period (543 million years ago) for the massive expansion in plant and animal species to take place.

This session looks at different aspects of the evolution of life on Earth: how it began, where it began and when it began.


The first spark of life

The generally accepted sequence of events that led to life presumably started with simple molecules. As has been discussed, these included materials brought to Earth by asteroids and comets. The energy needed to kickstart any reactions might have been generated by the regular bombardment of comets and asteroids, which fuelled reactions between atmospheric gases. Subsequent cooling caused the gaseous molecules to dissolve in water droplets that rained down in the oceans. The new watery environment allowed molecules to be built up into more complex organic compounds.


Where did life begin?


DNA is a repeating pattern of four components (bases)--adenine (A), cytosine (C), guanine (G) and thymine (T)--attached to a long chain of alternating sugar and phosphate groups. The bases are matched in pairs: A with T, and C with G. One suggestion for how DNA, and therefore life, came about from the simple molecules found on early Earth is via the single-stranded intermediary molecule, RNA, which is also made from a backbone of phosphate and sugar groups plus four bases: A, C, G and uracil (U). When an electric current, such as lightning, is applied to mixtures of carbon dioxide, carbon monoxide and ammonia (simple molecules found on the early Earth), hydrogen cyanide, cyanoacetylene and formaldehyde are produced. These molecules can be synthesised into the four bases that make up RNA, the precursor of DNA.

Until recently, scientists thought that the likely energy source for the reactions that led to the creation of life was the sun. This implies that organisms originated in surface waters and that the earliest lifeforms relied on photosynthesis, whereby energy from the sun converts atmospheric carbon dioxide into sugar and carbohydrates, releasing oxygen. However, the discovery of hydrothermal vents on the deep ocean floor and their associated lifeforms, has opened up the possibility that life may have emerged at depth. It is too dark here for photosynthesis to occur so organisms survive by chemosynthesis, whereby energy is derived from chemical reactions.

Clouds formed by organic-rich droplets thrown up from the ocean surface have also been suggested as a possible location for early life. Up in the atmosphere, reactions between the organic compounds could have been driven by solar energy to produce more complex molecules.

Wherever life began and whatever type of mechanism drove the process, simple molecules were built up into more complex molecules. The next steps in the evolution of life were the ability to self-replicate, development of an energy conversion process (or metabolism) to drive replication and a membrane to isolate the molecule from the surrounding medium. It is not clear which of these items came first or if they evolved in parallel.

All life today is based on DNA (deoxyribonucleic acid). Many details are still absent from our understanding of how DNA might have come about, but the related molecule, RNA (ribonucleic acid), was probably a significant intermediary in the process. It is hoped that advances in the mapping of the human genome will further the understanding of the origins of life.


What is life?

It is useful to define 'life' as 'the sum of all the activities of a plant or an animal' where activities are respiration, reproduction, nutrition, excretion, locomotion, growth and reaction to external stimuli. But clarity of this definition is not always straightforward to apply: for example, fire takes in oxygen, grows, moves, feeds, gives out heat, and reacts to external stimuli like wind, but is not 'alive' in the sense taken to apply to organisms. Therefore the ability to evolve and adapt to change offers a fuller description of 'life', of something that is 'living'.


When did life begin?

Up until about 4 billion years ago, the Earth was too unstable for life to take hold because of heavy bombardment by asteroids and comets. Although no fossils remain, rocks in the 3.85 billion-year-old Isua Complex in West Greenland seem to preserve a chemical signature indicating the presence of biological material. Simple fossils (chains of cells that can be compared to bacteria living today) have been found in 3.5 billion-year-old rocks. However it was not until the start of the Cambrian period, 543 million years ago, that the first proper expansion of plant and animal life occurred. The evolutionary pathway from the earliest simple-celled organisms to primates and humans is easy to follow by examining the structure of the molecules that make up the forms of life.

After its formation, the Earth gradually cooled and oceans formed. Its atmosphere became dominated by carbon dioxide, and these conditions led to the development of a group of organisms known as cyanobacteria. These organisms photosynthesised--took carbon dioxide in from the atmosphere and gave oxygen out. The balance of gases in the atmosphere changed; oxygen gradually built up with the photosynthesising activity of the bacteria, and carbon dioxide abundance decreased, mostly by the formation of limestones, and by weathering of silicate rocks. However, the deep ocean was still poor in oxygen and soaked up much of the newly available oxygen reserves. Eventually, though, oxygen began to accumulate above water too and around 2 billion years ago there was a sudden increase in the amount of oxygen in the atmosphere. Conditions were then ripe for the establishment of life.