The Cosmic Calendar scales the history of the Universe into a single year, starting with the Big Bang at an estimated 13.8 billion years ago set at January 1st at midnight, and ending with this very second as the final, tiniest increment of time on December 31st. Much of the cosmic year passes before we actually reach the formation of Earth and appearance of the oldest known rocks within the first cosmic week of September, or 4 billion years ago. In contrast, the earliest hominids (10-12 million years ago) did not appear until the early afternoon of December 31st. Domestication of fire (400,000 years ago) about 15 cosmic minutes ago, and all of modern civilization (within 500 years) is compressed within the last second.
The Cosmic Calendar is a useful tool, because it is really, really difficult to comprehend what a billion years means. By looking at the calendar, we realize that from a chronological aspect, human civilization has been incredibly insignificant. Looking at it a different way, we see that we are here because a combination and sequence of physical, chemical, and biological changes has allowed for ‘us’ to happen.
Let’s look at early Earth, about 3.5 billion years ago. The atmosphere consisted mainly of carbon dioxide (CO2), ammonia (NH3), and methane (CH4), with just a trace amount of oxygen. A Google Image search of the phrase “Early Earth” shows artist renditions featuring plenty of volcanoes and lightning. This is no place for humans, much less plants, reptiles, fish, or the nuclear fallout-surviving cockroach.
It is in this environment that the first biomolecules appeared. There are a few different theories as to what drove their formation (Lightning! Heat? meteorite impact??) but either way, the first, simple bacterial cells probably formed about this time.
Now, one of the major themes in ecology is the “ecological niche,” which is defined as a specific, specialized role that an organism has in its habitat. No two species can occupy the same niche; one will almost always outcompete the other.
As early Earth had abundant carbon dioxide, a life form appeared to occupy this niche, taking the abundant CO2 out of the air, and converting it to sugar and free oxygen.
Sound familiar? This is photosynthesis.
These life forms, called cyanobacteria, or photosynthetic bacteria, were literal catalysts for atmospheric and oceanic change on Earth.1 The abundant amount of sunlight and carbon dioxide allowed for a massive increase and diversification of cyanobacteria, continuously taking up CO2, releasing oxygen, and being buried in sediment. At first, this increase in free oxygen had little effect on the composition of the atmosphere, due to the presence of compounds that can take up oxygen (oxygen sinks) such as hydrogen sulfide, or iron (converted to iron oxide). After about 1.5 billion years, these sinks began to fill up, resulting in the increasing accumulation of free oxygen in the atmosphere.2
This massive change in the composition of the atmosphere is known as the Great Oxygenation Event. It has also been called the Oxygen Catastrophe, Oxygen Crisis, even Oxygen Holocaust.
If these names sound a bit dramatic, it’s because it was an extremely dramatic event. As humans, we see the Oxygenation Event as a positive thing, as it made possible the development of higher plants and animals. However, oxygen was highly toxic to the abundant anaerobic bacteria living at this time, causing a massive die-off and retreat to oxygen-poor environments.
There was another consequence of the “rapid” (remember, we are still on the scale of millions of years) and overwhelming change of the composition of Earth’s atmosphere. It is well-known that CO2 and methane traps sunlight in the atmosphere to maintain a warm temperature, much like a greenhouse. Removal of carbon dioxide and oxidation of methane from the atmosphere took away the greenhouse effect, which resulted in a massive drop in temperature around 2.3 billion years ago, ushering in a complete snowball Earth-type ice age that lasted for several million years.
The exact dates and duration of these events is disputed,2 for example, opinions differ on the rates and oxygen concentrations, the major driving forces that resulted in the saturation of oxygen sinks, and the number of Snowball Earth events that occurred.
There are a few takeaways here.
1) We commonly hear the phrase “Greenhouse Effect” in a negative connotation. We do need some greenhouse gases to trap enough heat in the atmosphere so that Earth does not freeze over. But, please note that by the time humans came around, we likely had a sufficient greenhouse effect for our survival, so please do not use this as an argument for burning more fossil fuels.
2) The molecules that make up terrafirma, the ocean, and atmosphere are continuously interacting and reacting with one another. The fact that a buildup of free oxygen in the atmosphere caused a domino effect that resulted in Earth being a snowball for millions of years has suggested that we not underestimate the effect of even a small change in atmospheric composition. No matter what your elected officials tell you, CO2 is not just a “harmless plant food.”3
3) If we look at the current state of affairs, we are literally reversing the Great Oxygenation Event. By burning fossil fuels, we are taking carbon from the ground, and releasing massive amounts of carbon dioxide into the atmosphere. While some greenhouse effect is necessary, too much is what is causing the rising global temperatures that are wreaking havoc on our polar ice caps, and this leads to many, many things which threaten our livelihoods.
I still maintain that the rise of cyanobacteria and the resulting changes that occurred were a good thing, since the most recent result was the development of human civilization. On the other hand, the Great Oxygenation Event did cause a mass extinction, killing off multitudes of critters that could not tolerate oxygen.
Mass extinctions are a common, regular occurrence throughout Earth’s history. The GOE was probably the earliest known mass extinction. There was a particularly devastating extinction at the end of the Permian Era (252 million years ago), which was also called the Great Dying. By the start of the subsequent Triassic period, about 96% of sea critters had died out, along with 70% of terrestrial critters.4 Probably the most famous mass extinction occurred at the Triassic-Jurassic period, which we know as that time in which all the dinosaurs died.
We are currently in another mass extinction, which is being called the Holocene. There is overwhelming evidence that it is human-made, although for some reason politicians want to argue over that.
A rather Circle-of-Life way of looking at Earth is the Gaia Hypothesis, which states that Earth is comprised of synergistic, self-regulating components that work as a whole to maintain life on the planet. It’s a pretty neat way of looking at things. Simply put, all of the organisms, whether it be plants, animals, microbes, and fungi require the action of each other for the maintenance of optimal conditions.
Along these lines, there is a second hypothesis that takes into account all of these mass extinctions, called the Medea Hypothesis. Medea was a figure in Greek mythology who killed her own children to get back at her unfaithful husband. I’m not quite sure who is the unfaithful husband in this scenario, but the hypothesis states that Earth will every so often rid itself of higher forms of life, and return to a state where only single-celled organisms exist.5
That brings us to our final takeaway:
4) By figuring out what has happened in Earth’s history, particularly what has shaped the planet for colonization by humans, we can develop strategies to either maintain control of these optimal “Earth” conditions, or use this knowledge to Terraform (make Earth-like) other planets. We will need this knowledge if in case we do end up mucking up Earth for human survival, or, that one day about 5 billion years down the road in which the sun will burn out, swell up, and engulf Mercury, Venus, and Earth. Mars will probably get away, the lucky bastard.
On a last note, the answer to removal of excess CO2 and methane is not to introduce massive quantities of cyanobacteria into the environment. Cyanobacteria are quite toxic, and tend to spoil important ecosystems. 6 Don’t do that.
1 Water: The Vital Fluid":http://eesc.columbia.edu/courses/ees/lithosphere/hays_tutorial_3/atm.htm
2 Holland, HD. 2006. The oxygenation of the atmosphere and oceans. Philosophical Transactions of The Royal Society. 361:903-915
4 Sahney, S., Benton, MJ. 2008. Recovery from the most profound mass extinction of all time. Proceedings of the Royal Society 275:759-765
5 Ward, P. 2008. The Medea Hypothesis: Is Life On Earth Ultimately Self-Destructive? ISBN: 781400829880