During this course we will repeatedly return to the unanswered question of the Origin of Life. We do this because:
- It's the Mother of All Biology Questions
- It requires that we define Life
- It encourages us to appreciate the diversity of Life's own solutions to a core "Design Challenge", the challenge in this case being- How to harvest the energy required to build Life?
Discussion of Origins requires that we understand (to the extent that geologists can tell us) what conditions were like on Earth when life first arose. Life is based on the ability to collect energy by tapping into chemical (including photochemical) reactions. Scientists studying origins and evolution need to understand how the chemistry of Earth, particularly of the Earth's atmosphere, has changed during the approx. 4 billion years since the solidification of this planet (or at least of its crust). Interestingly, Life itself has changed the chemistry of the Earth's atmosphere radically, and in turn Life has expanded its capacity to acquire and store energy in response to these Life-generated changes. If you're interested in more detail, you might start with the very nice summary of the history of our planet's atmosphere at the website for Prof. Perry Samson's course on global climate change at the University of Michigan.
The Earth and its moon are thought to be about 4.54 billion years old. This estimate is based on evidence from radiometric dating of meteorite material, together with other substrate material from Earth and the moon. Earth has an unusually large amount of water for an inner solar system planet, and it is thought that this water was brought to a young Earth through a collision with a characteristically water-rich outer solar system planetoid, named Theia. This collision pulverized both planets, which re-formed as our current Earth and its unusually large moon. This Earth, at first, lost its hydrogen-rich atmosphere to space, but these gasses were replaced through emission from deep within the Earth- these emissions continue today.
Earth's atmosphere changed radically during its history, from a reducing, H2-rich chemistry, to an oxidizing, O2-rich chemistry. As we'll see, Life needs to extract energy in different ways from these different environments; they each provide their own challenges. The temperature of the Earth has also changed radically ranging from extremely hot (at which point we assume there was no Life, as Life requires liquid water, but perhaps this assumption reflects our limited imagination) to a Snowball Earth, entirely encased in ice (though there was liquid water beneath that ice).Evidence indicates that during the first two billion years of Earth’s existence, the atmosphere was anoxic, meaning that there was no molecular oxygen (O2 ...there was of course oxygen, the atom, but these atoms were tied up in other chemical forms). Therefore, only those organisms that can grow without O2 — anaerobic organisms — were able to live. Both chemo-autotrophic organisms (organisms that make use of chemical reactions occurring in their environment to power their growth) and heterotrophic organisms (organisms that break down complex chemicals to acquire energy) rapidly evolved once liquid water was present. Photoautotrophic organisms that convert solar energy into chemical energy appeared within one billion years of the formation of Earth. The cyanobacteria, also known as blue-green algae, evolved from simpler photoautotrophs one billion years later. Cyanobacteria, still with us today, are able to acquire energy from sunlight and water, producing O2 as a waste product of photosynthesis. This waste product changed the nature of the planet, and the nature of life itself. Increased atmospheric oxygen allowed the development of more efficient O2-utilizing catabolic pathways. It also opened up the land to increased colonization, because some O2 is converted into O3 (ozone) and ozone effectively absorbs the ultraviolet light that would otherwise cause lethal damage to DNA. Ultimately, the increase in atmospheric O2 concentrations allowed the evolution of other, more complex life forms.
A nice summary of the history of O2 on Earth, try the Wikipedia site Geological History of Oxygen. You'll find this is a Biology story just as much as it is a Geology story.