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The Development of Life on Earth

This lecture examines how life on Earth first developed, and how this life dramatically changed the conditions of Earth's primordial surface. Some of the material is covered in Universe, Sections 8.5 and 30.1.

  1. Definition of Life
  2. Competing Theories
  3. The Primordial Earth
  4. The Formation of Chemical Building Blocks
  5. The Formation of Macromolecules
  6. The Formation of Prebionts
  7. The Formation of Prokaryotic Organisms
  8. The Evolution of Autotrophs
  9. The Evolution of Aerobic Organisms
  10. The Evolution of Eukaryotic Cells
  11. The Present-Day Atmosphere

Definition of Life

To discuss the development of life, it is first necessary to define what life is. This is a difficult task, since living organisms exhibit so many diverse properties and behaviours. However, a simple working definition, which encompasses most organisms, is as follows:

  1. Living organisms can react to their environments and heal themselves when damaged.
  2. Living organisms can grow by taking nourishment from their surroundings, and processing it into energy.
  3. Living organism can reproduce, passing along some of their characteristics.
  4. Living organism have the capacity for genetic change, allowing them to evolve.

Competing Theories

The two main competing theories for the development of terrestrial life are chemosynthesis and panspermia. The chemosynthesis theory maintains that life formed by the progressive assembly, on Earth, of more- and more-complex organic molecules and structures, until a point was reached where these molecules and structures amounted to living organisms.

In contrast, the panspermia theory maintains that the same assembly took place, but elsewhere in the Universe: either in space, or on the surface of another planet. Some mechanism (e.g., comets) was then responsible for seeding Earth with life or its precursor molecules.

Of the two theories, chemosynthesis is the preferred one, since panspermia appears to have significant flaws. This lecture therefore focuses on how chemosynthesis accounts for life on Earth. Panspermia will be covered in the next lecture.

The Primordial Earth

After its formation from planetesimals (see [link:diploma-1|Lecture 1]), the Earth would have undergone chemical differentiation, where the heavier elements sink to the core and the lighter elements rise to the surface. Among these lighter elements were traces of hydrogen and helium, which would have given rise to a thin primordial atmosphere.

Bombardment of primordial Earth

Bombardment of primordial Earth

Since Earth's gravity is too low for it to retain hydrogen and helium, these gasses would quickly have evaporated into space, leaving a bare rocky globe with no atmosphere or oceans. However, the contraction of the Earth under its own gravity, plus the decay of radioactive elements and bombardment by meteorites, would have then led to significant volcanic activity. This activity forced out gasses from the interior to form a dense evolutionary atmosphere, comprised mainly of water vapour, carbon dioxide and nitrogen.

The Formation of Chemical Building Blocks

As the Earth cooled, much of the atmospheric water would have condensed and fallen as rain, creating large oceans. Dissolved in the rain was carbon dioxide, which formated of carbonate rocks such as limestone and and marble. Through this process, most of the carbon dioxide was removed from the atmosphere, allowing the Earth to escape from a possible runaway greenhouse effect (see [link:diploma-2|Lecture 2]).

The next step in the development of life was the formation of simple organic molecules. In a famous experiment conducted in 1952, Stanley Miller and Harold Urey exposed a mixture of gaseous hydrogen, ammonia, methane and water to an electrical arc for a week. At the end of the experiment, the reaction chamber was coated with a reddish-brown rich in amino acids and other compounds essential to life.

The Miller-Urey experiment

The Miller-Urey experiment

The Miller-Urey experiment demonstrated how lightning may have converted the evolutionary atmosphere into a living atmosphere, rich in the chemical building blocks of life. However, it is important to understand that the experiment did not create life! A number of further steps, which have not yet been demonstrated experimentally, are required before life is formed.

The Formation of Macromolecules

After the formation of the amino acids and other building blocks, and their subsequent solution in liquid water, various processes (such as adsorption on clay particles, or confinement in evaporating pools) would have conspired to concentrate these compounds. Under the influence of an energy source (such as UV light or heat), the concentrated compounds would have combined to form large macromolecules, such as polypeptides (precursors of proteins) and polynucleotides (precursors of DNA).

The Formation of Prebionts

Once macromolecules had formed, the next step in the development of life would have involved their organization into bodies with definite shapes and chemical properties. One example is coacervate droplets, which may be the early ancestors of cells. These coacervates consist of macromolecules surrounded by a shell of water molecules, whose rigid orientation makes them form a primitive membrane. This membrane is highly selective, allowing only certain molecules to pass though; it therefore creates a sheltered chamber in which complex chemical reactions can develop. Have a look at http://www.indiana.edu/~ensiweb/lessons/coacerv.html for a description of how coacervates can be created in the lab.

Microscope image of coacervates

Microscope image of coacervates

The Formation of Prokaryotic Organisms

With ever-more complex reactions taking place in prebionts, a point was reached where self-replicating molecules were formed. One example is the nucleic acids, such as DNA and RNA. These molecules have the ability to copy themselves, and therefore act as information stores.

Due to random mutations occurring during the copying process, the appearance of self-replicating molecules meant that the prebionts began to evolve through the process of natural selection. Only those prebionts which were able to make the best use of the available sources of energy and raw materials were able to survive and produce a new generation of prebionts, containing the genetic information of their own "parents".

At the point, the prebionts had reached a level of advancement which amounted to living organisms (albeit primitive). These single-celled organisms were prokaryotic, meaning that they lacked an inner membrane around a nucleus of genetic material. They were much like present-day bacteria.

The Evolution of Autotrophs

The first cells were heterotrophs, meaning that they obtained their energy and raw materials (i.e., food) from their surroundings. Early on in their existence, the supply of these resources would have run short, amounting to a famine. This famine exerted extreme evolutionary pressure on the heterotrophs, leading quite quickly to the development of cells which were able to produce their own food via photosynthesis.

These new autotrophs (meaning that they create their own food, rather than relying on their surroundings) would have at first relied on a variant of photosynthesis based around hydrogen sulphide. Unfortunately, the supply of hydrogen sulphide is rather limited on Earth, being found only around areas of volcanic activity. Therefore, some autotrophs (the cyanobacteria) subsequently made the leap to using water instead, which is of course in great abundance.

Colonies of cyanobacteria

Colonies of cyanobacteria

The Evolution of Aerobic Organisms

When photosynthesis is based around water, it produces a significant by-product: oxygen. Since oxygen was highly toxic to the cyanobacteria producing it, they were forced to evolve means of protecting themselves from it, primarily by excreting it as a gas. Their success in this led to the steady pumping of oxygen into the Earth's atmosphere.

Initially, the oxygen would have reacted with surface minerals to create oxides. This would have gone on until about 2 billion years ago (see Universe, fig. 8.22), when all of the available minerals were already oxidized. At this juncture, the levels of atmospheric oxygen would have begun to rise, and a new type of heterotrophic life evolved to take advantage of the oxygen as an energy source: the aerobic respirators.

The Evolution of Eukaryotic Cells

Around 1.5 billion years ago, eukaryotic organisms first appeared. Unlike prokaryotic organisms, these possessed inner membranes around a nucleus of DNA, and also contained sophisticated organelles such as mitochondria (for aerobic respiration) and chloroplasts (for photosynthesis). Subsequently, the eukaryotic cells developed into specialized colonies, and provided the basis for all multicellular life known today.

A comparison of prokaryotic and eukaryotic organisms

A comparison of prokaryotic and eukaryotic organisms

The Present-Day Atmosphere

Until about 400 million years ago, the levels of oxygen in the atmosphere were steadily growing. However, at this point the amount of oxygen produced by the photosynthetic autotrophs was balanced by the amount consumed by the aerobic heterotrophs, and the growth stopped. Since then, the composition of the Earth's atmosphere has remained relatively unchanged. This present-day atmosphere has a composition of about 20% oxygen, 78% nitrogen, and small amounts of water vapour and carbon dioxide.


Updated 2009-10-13 12:28:14