The theory of evolution explains how organisms change over the generations and what guides that change. Using the theory of evolution, biologists can understand why living things have the characteristics they have and how different characteristics may change in future generations. The explanatory power of evolution is so important to biology that it ends up being an underlying theme of much of the SAT Subject Test.
Cell theory says that all cells arise from previous cells. Why is that? Why can't cells just form on their own if you mix the right molecules together? The problem is that even the simplest prokaryotic cells are amazingly complicated and finely structured organisms, and organization like that doesn't just pop up on its own. That means every new cell needs to be built by a previous cell. But, wait a minute . . . this can't be completely right because we know that life on earth had to start sometime in the past, and whatever the first cells were, they didn't come from previous cells. So how did the first cells arise? The answer is that nobody really knows, because nobody was around to see it happen 4 billion years ago, and nobody's seen it happen again since then. Nevertheless, biologists have some interesting guesses (as they always do), and the SAT Subject Test just may ask you about the most famous theory (as it always does).
Early atmosphere
Instead of the nice, fresh mixture of oxygen (O2) and nitrogen (N2) that we have today, the early earth atmosphere had a toxic mixture of methane, hydrogen gas, and ammonia. So how could life arise in that? Well, a scientist tried to answer that question by re-creating the conditions of the early earth in a big beaker in his lab. Amazingly, he found that organic molecules like sugars and amino acids formed automatically, and these molecules are part of what go into making cells. The results from this experiment form the basis of the current theory for the origins of life.
First cells
We now know that the building blocks of life were present in the oceans of the early earth, forming an organic mixture called the primordial soup. As time went on, the molecules of the primordial soup got more and more complex and they engaged in complex chemical reactions. In some of these reactions, big molecules like proteins and nucleic acids could actually copy themselves. Then, when the right kinds of self-copying molecules were trapped inside of the right kind of oil droplets, the first prokaryotic cells were formed!
These first cells could reproduce themselves and were therefore subject to evolution by natural selection. They were also able to use the organic molecules floating around in the primordial soup as fuel and nutrients. To break down their fuel, they used anaerobic respiration.
A change of atmosphere
Anaerobic respiration tends to produce carbon dioxide (CO2), which is a molecule that was not present in the atmosphere before then. Once CO2 was around (after a few hundred million years), other cells could use it to perform photosynthesis. Photosynthesis produces free oxygen (O2), another molecule that had never been around before. Once photosynthesis had produced enough O2, the atmosphere started to become a lot more like it is today. The O2 allowed complex eukaryotic cells to emerge using aerobic respiration, which is a very efficient way to break down fuel. These eukaryotes gradually became more complex and eventually began forming multicellular organisms, until we finally end up with all of the varieties of life we see on earth today! Okay, that's the whole story of four billion years of evolution in a few paragraphs. Whew.
To test your understanding of the workings of billions of years of evolution, the SAT Subject Test may present questions like the following.
Which of the following was not a component of the early earth atmosphere?
- ammonia (NH3)
- oxygen (O2)
- water (H2O)
- methane (CH3)
- hydrogen (H2)
This is a tough one if you aren't good at straight memorization. One thing you may remember is that O2 is a big part of our atmosphere today, and the atmosphere of the early earth was completely different than it is now. Also, the first O2 molecules weren't produced until photosynthesis finally got going, and that didn't happen until life had been around for a long time. So, the correct answer is (B). Water is always around on the earth, and the other gasses were all toxic components of the early atmosphere that are found only in very small amounts in the atmosphere today.
Which is the correct sequence for evolution of life on earth?
- eukaryotic cells emerge
- anaerobic respiration develops
- prokaryotic cells emerge
- pre-biotic primordial soup
- photosynthesis develops
- I, III, V, II, IV
- IV, II, III, V, I
- IV, III, II, V, I
- II, III, V, I, IV
- IV, II, III, I, V
The pre-biotic soup (IV) has to be first, because pre-biotic means "before life," so (A) and (D) can be crossed out right away. The first cells were simple prokaryotes, so III must come next, which eliminates (B). Anaerobic respiration paved the way for photosynthesis, and then that allowed complex eukaryotic cells to emerged. That means (C) must be the correct answer.