"Joy and pleasure are as real as pain and sorrow and one must learn what they have to teach. . . ." -- Sean Russell, from Gatherer of Clouds

"If you're not having fun, you're not doing it right." -- Helyn D. Goldenberg

"I love you and I'm not afraid." -- Evanescence, "My Last Breath"

“If I hear ‘not allowed’ much oftener,” said Sam, “I’m going to get angry.” -- J.R.R. Tolkien, from Lord of the Rings

Saturday, July 16, 2016

Saturday Science: Earth: A Biography: Prokaryotes and Beyond

To backtrack a bit, the formation of the Earth took place at the beginning of the Hadean Eon (from the Greek "Hades"), which lasted from about 4.5 to 4 billion years ago. Although it's generally considered that the surface of the Earth, such as it was, was extremely unstable, with major seismic activity, strong upwelling of magma and subduction of what surface managed to form, there is evidence that the continents had, indeed, started to form, that there were seas, and possibly even rivers -- in effect, Earth was more user friendly earlier than we had thought. There are pieces of zircon from Australia that have been dated to 4.1 billion years ago, and the oldest extant rocks are found in Quebec and go back to 4.2-4.3 billion years ago. (There even seems to be evidence that the moon formed during this period, but since I'm not doing a biography of the moon, I'm not going to worry about it.)

Fast forward to the Archean Eon (which is actually the first division of the Precambrian Era, which might be a little more familiar), which started about 4 billion years ago, and we have life: the earliest evidence suggests living organisms from at least 3.5 billion years ago, and there is some evidence that life actually began as early as 4.1 billion years ago -- although that's still a big maybe. (That bit of zircon turned out to be quite important -- and fairly controversial.) The image at left should give you some idea of what the world looked like at that point: mostly water, not much land.

The first living organisms were prokaryotes (from the Greek words meaning "before the nut" -- that is, before the nucleus). They have no discrete nucleus and no discrete organelles (mitochondria), although they do have an analogous structure, a ribosome. These produce proteins, much the same as the DNA and mithochondria in eukaryotes (which we'll get to in a bit). The "contents" -- DNA, proteins, metabolites, all the things that make them work -- are enclosed within the cell membrane without being separated into compartments. The prokaryotes fall into two groups, the archaea and bacteria. Both groups are still with us.

3500 million year old Apex Chert, Australia
containing the first fossil evidence of life on Earth
The archaea constitute a domain (the highest level of classification) and a kingdom. They were originally considered to be "extremophiles" confined to extreme environments, such as undersea thermal vents (remember those?) and highly alkaline lakes (as in California's Mono Lake or Africa's Lake Natron). They have, however, been discovered in much less hostile environments. Their ability to survive the extremes is due to the structure of their cell membrane, which I'm not going to get into in any detail here (see this if you're interested).

The really interesting part, aside from the fact that the Archea are still us (in environments as diverse as marshes, undersea vents, and the human gut) is that they are thought to be the ancestors of the eukaryotes -- organisms with nucleus and organelles bound by membranes.

The other major group among the first living organisms are the bacteria, which, as you well know, are also still with us. While they have some distinct differences from archea, they are alike in two very important ways: both reproduce asexually, by fission, and both allow for lateral gene transfer. That is, genetic material can be transferred between two different organisms, which sort of blows the idea of "species" as a group of organisms that produce viable offspring within the group but cannot produce viable (fertile) offspring outside the group. Think horses and donkeys: mules are sterile.

One of the most important things about bacteria at this stage of the game is that some of them started using colored pigments -- mostly chlorophyll, but there were others as well -- to convert sunlight into energy: combine carbon dioxide and water and through the action of chlorophyll as a catalyst, you create simple sugars, which you can then burn for energy. The reason this is important is because one of the byproducts -- the waste product, actually -- is oxygen.

Banded iron formation.  Photo via Britannica
Remember, up until the advent of cyanobacteria -- the ones who used photosynthesis -- there was no free oxygen. It was all bound up with hydrogen, carbon, nitrogen, what have you to form water, carbon dioxide, various acids, and the like. It took a while, but the advent of free oxygen, first in the oceans and then in the atmosphere, fundamentally changed the character of life on earth. It took several million years, because there was, for example, a lot of iron in the environment, which latched on to the oxygen molecules to form what are technically known as "banded iron deposits," or more commonly, "rust." The first evidence we have of this dates to about 3 billion years ago, so you can see that a) it took some time for cyanobacteria to develop and b) it took even longer for all that iron to turn to rust.

So, we have what I consider the first major extinction (although no one else seems to think of it that way) with the advent of free oxygen in the atmospere, which wiped out a large percentage of the single-celled organisms then existing (although some survived -- even today, we have anaerobic bacteria, which find oxygen toxic) and paved the way for life as we know it.

So, next time we'll talk about the eukaryotes, where they came from, and how they developed.

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