Overview [1]
Have you read the preceding page "Foreword"?
Scientists estimate that the Earth is approximately 4.6 billion years old, and that life probably began in the ocean at least 3.5 billion years ago. The original building blocks of life were the lighter elements, hydrogen and helium, formed in the aftermath of the big bang. To these were later added the heavier elements, including carbon, oxygen, hydrogen, nitrogen, sodium, iron, the product of supernova explosions successively absorbed by recycled stars. In time, these raw chemicals combined into molecules such as water (H2O), ammonia (NH3) and carbon dioxide (CO2), and then into simple organic molecules like methane (CH4). These organic molecules found an environment where they could react and become increasingly more complex, forming biomolecules, the molecules that make up living beings. Water (in its liquid phase due to warmth) was an essential ingredient of this process, the first steps of which were the formation of amino acids.
By a process which is not well understood, a set of self-sustaining chemical reactions capable of absorbing energy from the environment and of replication occurred. Maybe shallow pools (Darwin’s “warm little ponds”) provided a high enough concentration of chemicals; reactions started and molecules grew in complexity until they became a self-sustaining reaction network, extracting energy from the environment to keep on going. At some point a membrane made of fatty molecules surrounded the reacting chemicals, isolating them from the outside environment creating a primitive cell. Isolating these chemicals behind a semi-permeable membrane allowed reactions to thrive. This was the first prokaryote or cell without a nucleus. With increasing efficiency, the membrane allowed energy and nutrients to come in and waste to get out. Meanwhile, the genetic material inside the primitive cells replicated, leading to fast diversification. Blue-green algae and many bacteria are prokaryotes, primitive cells where the genetic material, the DNA used in their reproduction, is bundled into a coil without a membrane separating it from the rest of the cell. For nearly 2.3 billion years, life consisted of these single-celled microbes alone, albeit some organised in colonies.
Then, about 1.2 billion years ago, more complex multi-celled organisms appeared. We also have little understanding how this occurred, although we do know that it took close to 2 billion years. The most accepted view is that eukaryotes (cells with nuclei) developed from symbiotic alliances between prokaryotes (cells without nuclei). For example, mitochondria, the modern cells little engine, are believed to have been a separate organism in the distant past that was either eaten or absorbed by another cell. In eukaryotes (the more sophisticated cells like the ones in our body), an isolated nucleus houses the genetic material. Eukaryotes appeared towards the end of this period, when oxygen became more abundant in the atmosphere thanks to collective efforts of the photosynthetic blue-green algae. Sponges, the first multicellular organisms, appeared as early as 1.8 billion years ago. The crucial transition from single-celled to multicelled organisms - from our amoeba-like ancestors to sponges - happened for a number of improbable factors: most probably the increase in atmospheric oxygen between 2.7 and 2.2 billion years ago. An increase in the production of ozone due to the action of ultra-violet sunlight on the oxygen, without which we could not survive, occurred at the same time. [2]
After about 3 billion years after life’s first known traces, a transition took place from unicellular to multicellular organisms. As with the transition from prokaryotes to eukaryotes (from cells without nuclei to those with a nucleus), multicellular organisms possibly also evolved through symbiotic trial-and-error processes, as different kinds of unicellular organisms linked to each other (or ate each other) and became pluralistic in form and function. It remains difficult to understand how the different types of DNA incorporated into a single genome. As an alternative explanation, the Colonial Theory proposes that unicellular creatures grouped in colonies that slowly evolved into multicellular animals.
Many scientists propose that environmental changes in the Earth played a major role in accelerating the diversity of complex multicellular organisms that climaxed during the so-called Cambrian explosion, about 550 to 600 million years ago. Chief among them were the rapid increase in oxygen availability and the advent of plate tectonics and the consequent mixing of surface and ocean chemistry. Tectonics work as a global thermostat, recycling chemicals that help regulate the levels of carbon dioxide and keep the global temperature stable. Without it, surface water would not have remained liquid for billions of years and life, especially complex life, would have faced insurmountable obstacles.
After about 500 million years of evolving multicellular organisms, including many severe mass extinctions and climate changes, the first member of the genus Homo appeared in Africa some 4 million years ago. Intelligent life as we know it is less than 1 million years old and accounts for less than approximately 0.02 of the Earth’s history. [3]
Scientists estimate that the Earth is approximately 4.6 billion years old, and that life probably began in the ocean at least 3.5 billion years ago. The original building blocks of life were the lighter elements, hydrogen and helium, formed in the aftermath of the big bang. To these were later added the heavier elements, including carbon, oxygen, hydrogen, nitrogen, sodium, iron, the product of supernova explosions successively absorbed by recycled stars. In time, these raw chemicals combined into molecules such as water (H2O), ammonia (NH3) and carbon dioxide (CO2), and then into simple organic molecules like methane (CH4). These organic molecules found an environment where they could react and become increasingly more complex, forming biomolecules, the molecules that make up living beings. Water (in its liquid phase due to warmth) was an essential ingredient of this process, the first steps of which were the formation of amino acids.
By a process which is not well understood, a set of self-sustaining chemical reactions capable of absorbing energy from the environment and of replication occurred. Maybe shallow pools (Darwin’s “warm little ponds”) provided a high enough concentration of chemicals; reactions started and molecules grew in complexity until they became a self-sustaining reaction network, extracting energy from the environment to keep on going. At some point a membrane made of fatty molecules surrounded the reacting chemicals, isolating them from the outside environment creating a primitive cell. Isolating these chemicals behind a semi-permeable membrane allowed reactions to thrive. This was the first prokaryote or cell without a nucleus. With increasing efficiency, the membrane allowed energy and nutrients to come in and waste to get out. Meanwhile, the genetic material inside the primitive cells replicated, leading to fast diversification. Blue-green algae and many bacteria are prokaryotes, primitive cells where the genetic material, the DNA used in their reproduction, is bundled into a coil without a membrane separating it from the rest of the cell. For nearly 2.3 billion years, life consisted of these single-celled microbes alone, albeit some organised in colonies.
Then, about 1.2 billion years ago, more complex multi-celled organisms appeared. We also have little understanding how this occurred, although we do know that it took close to 2 billion years. The most accepted view is that eukaryotes (cells with nuclei) developed from symbiotic alliances between prokaryotes (cells without nuclei). For example, mitochondria, the modern cells little engine, are believed to have been a separate organism in the distant past that was either eaten or absorbed by another cell. In eukaryotes (the more sophisticated cells like the ones in our body), an isolated nucleus houses the genetic material. Eukaryotes appeared towards the end of this period, when oxygen became more abundant in the atmosphere thanks to collective efforts of the photosynthetic blue-green algae. Sponges, the first multicellular organisms, appeared as early as 1.8 billion years ago. The crucial transition from single-celled to multicelled organisms - from our amoeba-like ancestors to sponges - happened for a number of improbable factors: most probably the increase in atmospheric oxygen between 2.7 and 2.2 billion years ago. An increase in the production of ozone due to the action of ultra-violet sunlight on the oxygen, without which we could not survive, occurred at the same time. [2]
After about 3 billion years after life’s first known traces, a transition took place from unicellular to multicellular organisms. As with the transition from prokaryotes to eukaryotes (from cells without nuclei to those with a nucleus), multicellular organisms possibly also evolved through symbiotic trial-and-error processes, as different kinds of unicellular organisms linked to each other (or ate each other) and became pluralistic in form and function. It remains difficult to understand how the different types of DNA incorporated into a single genome. As an alternative explanation, the Colonial Theory proposes that unicellular creatures grouped in colonies that slowly evolved into multicellular animals.
Many scientists propose that environmental changes in the Earth played a major role in accelerating the diversity of complex multicellular organisms that climaxed during the so-called Cambrian explosion, about 550 to 600 million years ago. Chief among them were the rapid increase in oxygen availability and the advent of plate tectonics and the consequent mixing of surface and ocean chemistry. Tectonics work as a global thermostat, recycling chemicals that help regulate the levels of carbon dioxide and keep the global temperature stable. Without it, surface water would not have remained liquid for billions of years and life, especially complex life, would have faced insurmountable obstacles.
After about 500 million years of evolving multicellular organisms, including many severe mass extinctions and climate changes, the first member of the genus Homo appeared in Africa some 4 million years ago. Intelligent life as we know it is less than 1 million years old and accounts for less than approximately 0.02 of the Earth’s history. [3]
[1] There are two principal sources for this overview: Geoffrey Dawkins’ The Greatest Show on Earth – The Evidence for Evolution, Bantam Press, Great Britain, 2009, hereinafter referred to simply as Greatest Show or GS, and the Smithsonian Institute exhibition, Life and the Oceans, Smithsonian Natural History Museum, Washington DC, October 2010.
[2] Greatest Show, 183.
[3] Greatest Show, 235-237.
[2] Greatest Show, 183.
[3] Greatest Show, 235-237.