The timescale of life on Earth

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We've already talked at some length about how most of Biology today is comprized of microbes. It is worse than this - throughout most of Earths history, life has been entirely microbial!

Here is a copy of the geological time scale in it's usual format (click on it to download a larger version):

Geological scale
Geologic time scale, Geological Society of America

The problem with this view of geoloical time is that it is a logarithmic scale, and so dramatically emphasizes recent times over ancient times. It's done this way simply because we have a lot more information about recent geological time - the further back you go, the less we know and so the lower the resolution of our view.

In attempt to try to bring these time scales into a more realistic focus, here is the time scale of life events (very approximate) compared to the length of this classroom (~45ft):

Time

Scale Events
4.6 Bya 46ft Earth forms by accretion in the developing solar system
3.8 Bya 38 Oldest rocks - final sterilizing impacts
3.6 Bya 36 First apparent trace fossil bacteria
2.7 Bya 27 Trace oxygen appears in the atmosphere
2 Bya 20 The oxygen catastrophe - [O2] jumps to ~0.01atm (5% of modern level)
Snowball Earth
1.2 Bya 12 First colonial animals appear
700 Mya 7 First multicellular (but still microscopic) animals & plants appear
540 Mya 5'5" Cambrian explosion - all animal phyla appear at once
O2 jumps to essential modern levels
Beginning of the Paleozoic
430 Mya 4'4" Plants begin to colonize land
360 Mya 3'7" First trees appear - these are the ones we burn in our cars today
First insects appear
Amphibians invade land
245 Mya 2'5"

Permian extinction ~95% of species extinguished
Beginning of the Mesozoic - Age of Reptiles

65 Mya 6" Mass extinction of dinosaurs
Start of the Age of Mammals
3 Mya 1/3" First hominids (Australopithecus)
200,000 ya 0.024" (0.5mm) Homo sapiens appears
5000 ya 10um Agriculture & first civilizations appear
200 ya 500nm USA declared independence from England
20 ya 500Angstroms You were born
Cell phones invented

Notice that life seems to have originated and evolved into recognizable forms very quickly, within about 200 million years. After this, for the next 3 billion years, 80% of Earths history, life was entirely microbial. During this time the concentration of oxygen in the atmosphere was not high enough to be breathable. Remember this when people dismiss other planets for being "uninhabitable", or say that even if there could be life there, it would "only" be microscopic. This was true for this planet over most of its history. And, of course, microbes still rule the ecosystems; in the words of Stephen J Gould, a very famous paleontilogists and evolutionary biologist, "This is the Age of Bacteria - as it ever has been and always will be."

It is clear is that life on Earth has existed for at least 3.5 Bya, and probably more, although not before about 3.8 Bya because the large impacts that took place before then that would have vaporized the oceans and, in some cases, a fair amount of the crust of the planet. Before 3.8 Bya, the planet may have gone through alternating cycles of sterilizing heat from impacts, and crushing cold with completely frozen oceans.

In the last 3.8 billion years, life has survived many trials in the history of the planet, some of it's own making. Large impacts are now rare, and not of 'sterilizing' energy, but they do occur. The most famous is the K-T impact that probably played a role in the extinction of dinosaurs about 65Mya, but previous impacts were much larger, such as the Permian impact 240Mya that extinguished 95% of all macroscopic species.

The temperature of the planet has also changed, and seems to be less stable than previously thought. It seems likely that Earth has looked a lot like Europa, with completely frozen-over oceans, perhaps several times over the last 4 billion years, the last time about 0.6 Bya, just before the Cambrian explosion. The planet is in a delicate balance between the energy provided by the sun and how much of this is retained via greenhouse gases. Today, the major greenhouse gas is carbon dioxide, produced by photosynthesis, without which the Earth would freeze. Prior to about 2 Bya, however, methane was probably the major greenhouse gas, and this methane was apparently also the product of life (methanogenesis).

Another issue is atmospheric oxygen. Although we think of oxygen as an essential for life, high concentrations of oxygen are a recent feature of Earth's atmosphere. Oxygen concentrations before about 2.8 Bya are immeasurably small, less than 0.01% of the current level. Oxygen produced by photosynthesis was absorbed by the planet before 2 Bya, when the 'buffering' capacity was exhausted and the oxygen concentration in the air increased dramatically to as much as 2% - 10% of the current level. This has been described as the greatest climactic change ever to have occurred on Earth. Although animals need oxygen, it is not an innocuous compound - imagine if the Earth's atmosphere suddenly rose to 2% ammonia! Over most of the last 1 billion years, the oxygen concentration has risen about 10-fold to it's current level, most of this change occurring at about the time of the Cambrian explosion.