How the Evolution of Life Affected Early Earth Conditions

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll give particular attention to how early life changed Earth's atmosphere. We'll also talk a little about chemical evolution and the prebiotic soup.

Early Air

Where did the air come from?

Our planet is about 4.6 billion years old. Its atmosphere looked pretty different at the beginning. At first, Earth's atmosphere was probably dominated by hydrogen and helium gases. Later, it was dominated by carbon dioxide, carbon monoxide, ammonia, and other compounds. Oxygen wasn't even really a consideration in Earth's atmosphere for the first one or two billion years, although oxygen currently makes up about 21% of our atmosphere. So, where did it come from?

The very first atmosphere on Earth was probably dominated by hydrogen and helium gases. However, gravity was not strong enough to keep these very light molecules from drifting off into space.

Over time, Earth's atmosphere was dominated by carbon dioxide, sulfur dioxide, carbon monoxide, methane, ammonia, water, chlorine gas, nitrogen gas, and others. These would have been byproducts from early volcanic eruptions.

Chemical Evolution

Believe it or not, oxygen molecules themselves are pretty damaging. That's why it was much easier for life to evolve in an oxygen-poor environment.

In an atmosphere like that of early Earth's, free from the destructive power of oxygen, complex molecules that we think of as precursors to life would be able to form naturally.

That may sound weird, but we've actually managed to do this in a laboratory, way back in the 1950s. In the famous Miller-Urey experiment, scientists recreated early Earth's atmosphere. Within a week, they found organic compounds such as amino acids, fatty acids, and urea. Though our ideas of what early Earth's atmosphere was like have changed since Miller and Urey, subsequent experiments with updated assumptions have found the same thing.

Today, most scientists believe that oceans during this time period were a sort of prebiotic soup, full of molecules called depsipeptides, which can be combined to make proteins. These depsipeptides were believed to have been quite common in the early oceans. Without oxygen in the atmosphere to react with these molecules, and without bacteria to decay them, they simply would have hung around after they were formed.

Scientists have shown that when these precursor molecules are splashed onto hot rocks, they will combine to form protein-like structures.

As you can imagine, these compounds came in handy to the earliest life on earth. We believe that the earliest organisms that evolved were heterotrophs; that is, they had to consume their food. Luckily for them, the oceans in which they evolved were full of these organic compounds for them to eat.

What Happens When You Run Out?

Over time, life did pretty well. However, that meant that the nutrients found in early Earth's oceans started to get harder to find. At that point, an autotroph - an organism that could make its own food - would begin to have an advantage.

Photosynthesis is the most common form of autotrophy, although a few organisms use a related process called chemosynthesis. In modern photosynthesis, carbon dioxide and water molecules are converted into sugar and oxygen, with a light source used as energy.

That's a pretty complex reaction, though. Early photosynthesis was likely a little simpler. Though the details are still being worked out, many scientists believe that the earliest photosynthesizing organisms oxidized hydrogen gas into water, rather than oxidizing water to molecular oxygen. They also had precursors to chlorophyll, the green pigment used in modern photosynthesis. These earliest photosynthesizing organisms are preserved in the fossil record from about 3.4 billion years ago. There may be samples as old as 3.8 billion years, although some scientists dispute this.

The first photosynthesizing organisms that produced oxygen as a byproduct were cyanobacteria. Cyanobacteria appeared in the fossil record about 2.7 billion years ago. From there, cyanobacteria started slowly churning out oxygen. Because the first atmospheric oxygen reacted with iron, sulfur, and methane, it took about three or four hundred million years for cyanobacteria's most important product, oxygen, to begin appearing in the geologic record.

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