The Evolution of Life
It should be noted that the fossil evidence, dating back beyond 3.8 billion years, is almost non-existent and so some of this evidence is speculative. However, the latest advances in science are pushing the beginnings of life on Earth back beyond 3.8 billion years; although it might be more accurate to describe this type of life as `living matter`, because its existence appears to be little more than a chemical process. Even so, over eons of time, this living matter appears to have evolved to become evermore complex and diverse in its nature, although this interpretation of the general nature of evolution needs to be examined in more detail. However, before we can do this, lets try to get some appreciation for the process of evolution as a whole:
- Some 4.6 billion years ago, the Earth had started to solidify
within the solar system. However, radioactive and gravitational
heating of the Earth was still creating its interior structure,
as well as out-gassing lighter molecules, such as water, methane,
ammonia, hydrogen, nitrogen and carbon dioxide.
- By some 3.8 billion years ago, the crust of the Earth had solidified
and the oldest rocks found on Earth were being formed. At the same
time, condensation of water in the atmosphere was forming the first
oceans. Given that there is some fossil evidence that single-celled
prokaryotic organisms existed at this time, it appears that primitive
life appeared very quickly in the primordial oceans, at least, on
the timescales being discussed.
- This initial type of organisms is called a `heterotroph`
and is a class of life that feeds off other forms of life. These
organisms, which inhabited the primitive oceans, are believed to
have absorbed organic material, e.g. amino acids. When they died,
the organic material from which they were made, would break down
and be returned to the organic soup. The building blocks of life
that were creating these organisms, were also acting as a food source.
- Around 3 billion years ago, fossil evidence starts to suggest
that a new class of life called `autotrophs` was also evolving
in response to competitive pressure for resources. This adaptation
was capable of synthesising energy from inorganic material, which
allowed life to generate energy by absorbing carbon and sunlight.
As a result, CO2 started to be absorbed and oxygen was
released as a by-product. As a consequence, oxygen began to build
up, which in-turn created further environmental changes to which
life had to respond.
- This symbiotic relationship would suggest that the evolution
of Earth's environment was linked to the very life it was causing
to evolve. Over time, organisms continued to evolve in order to
take advantage of these new environments, as they emerged. However,
natural resources were always limited and life had to literally
fight for survival from the outset. Pathogens are an example that
were capable of killing single cell hosts in order to extract resources
required for their own continued survival, before moving onto the
next host after multiplying.
- Another significant step in evolution were eukaryotic cell organisms
called protocists. These organisms were significant because they
possessed a cell nucleus, which contained the genetic information
about the cell and the organism. Life was starting to evolve a cell
structure. Mitochondria are a key element present in both animal
and plant cells, which suggests it evolved before the evolutionary
split into plants and animals.
- Still, it would take nearly 3 billion years for life to evolve
from primitive one-celled organisms, as just outlined, to become
multi-cellular organisms in the form of sponges. At this time, clusters
of cells started to take on specific functions, which simply helped
in the survival of the larger organism.
- Another milestone in early evolution was the development of sexual reproduction. Previously, the cell division that produced a new organism was based on cell replication that caused mutations, i.e. errors, to drive evolution. In contrast, sexual reproduction allowed genetic information to be inherited from both 'parents', which although different, were both proven blueprints of survival, rather than just random errors. After the evolution of sexual reproduction, the rate and diversity of genetic variation began to increase to compete for every niche in the ecosystem.
These initial steps would take nearly four billion years, from the formation of Earth within the solar system to the early appearance of multi-cell organisms. We have now arrived at the start of what is now called the Cambrian period. At this point, some 500 million years ago, the stage was set for the diversity of life on Earth to explode. However, before continuing with the story of evolution, some scientists are now taking issue with the interpretation that the process of evolution is geared towards complexity and that humanity and mammals were the winner in this selective process. However, before returning to this issue, lets try to detail the timeline of evolution:
- Archaezoic Era: 2500-4500 million years ago
Over the totality of this era, microbial life flourished in the primordial oceans. One type of organism, the cyanobacteria (blue-green algae) started to produce oxygen as a metabolic by-product. The eventual build-up of this highly reactive gas was to eventually prove fatal to many life forms and was a major catalyst for change.
- Proterzoic Era: 545-2500 million years ago
This era saw the atmosphere stabilise to near present-day levels that enabled the rise of aerobic life. The original anaerobic inhabitants of the Earth had to retreat into more restricted habitats. The more complex eukaryotic cell organisms benefited from the increase in oxygen. Multi-celled organisms had started to appear between 600-1600 million years ago.
- Paleozoic Era: 245-545 million years ago
This era is sub-divided into a number of distinct periods.
|Period||Millions of Years Ago|
By the start of this era, the stage was set and life seems to have almost exploded in both complexity and diversity. Atmospheric oxygen was approaching present-day levels and generating the ozone shield that would screen out ultraviolet radiation. This change to the ecosystem would allow complex life to live in the shallow seas and finally on land. By the Cambrian period, trilobites were dominant, with lichen and mosses developing. The Ordovian period saw primitive vertebrates emerge along with fungi and plants. During the Silurian and Devonian periods, bony fish led to amphibians that started the colonization of the land along with insects. Later, in the Carboniferous and Permian period, the first reptiles started to appear. However, the Paleozoic era was brought to an abrupt end by mass-extinction, perhaps the most severe extinction the planet has seen.
- Mesozoic Era: 65-245 million years ago
This era is also sub-divided into a number of distinct periods.
|Period||Millions of Years Ago|
This era was the dawn of the great age of the dinosaurs. The generalized reptiles of the Triassic gave way to the dinosaurs and terrestrial fauna, which the Earth has not seen before or since. The dinosaurs ruled the land and a diversity of sea-reptiles ruled the oceans. In addition, Pterosaurs were taking to the sky, followed by the birds. Although mammals existed, they were still small and insignificant. During this era, evolution continued to produce new insects, fishes and finally flowering plants. Again, as with the Paleozoic Era, this era was marked by another mass extinction, which effectively terminated the age of the dinosaurs.
- Cenozoic Era: 2-65 million years ago
This era is sub-divided into the following periods.
|Period||Millions of Years Ago|
Following the extinction of the dinosaurs at the end of the Mesozoic era, space in the ecosystem emerged that allowed the mammals to evolve. The appearance of grass gave rise to a broad range of grazing herbivores and hunting carnivores. However, possibly more significant to our story, was the emergence of anthropoid apes that culminated in the hominids of Africa. Finally, after nearly 3.8 billion years, Homo Erectus, Neanderthal and Cro Magnon had started to learn how to use stone tools and make fire. However, the birth of any meaningful civilisations would still have to wait for Homo Sapien to appear, possibly only some 200-300 thousand years ago, which is virtually no time at all in terms of the geological time being discussed.