The age of the Earth

Although earlier scientists didn't have the tools to date the age of the Earth as we do today, they understood that the lower bands (and the fossils in them) were older than the higher bands and their fossils. Still, Darwin had no idea how immensely old the Earth was or how long the evolutionary process had been going on. Even when people began to understand that the world was quite a bit older than previously thought, they couldn't give an exact age to it.

Dating the age of the Earth: Radioisotope dating

Scientists know that the Earth is about 4.5 billion years old by using radioisotope dating techniques. To understand how this process works, you need to know a little bit about atoms and isotopes. For those who need little refresher course on basic chemistry, think of water, or H2O. The H and the O refer to hydrogen and oxygen, the two atoms that make up water. As the notation indicates, water consists of one molecule of oxygen and two molecules of hydrogen.

Often, any one atom has several different forms, which are referred to as isotopes. Atoms are made up of electrons, protons, and neutrons, and the number of electrons and protons determines the type of atom. Hydrogen, for example, has one electron and one proton. Sometimes, it also has a neutron. The term heavy water refers to water in which each hydrogen atom has a neutron. This isotope of hydrogen is also called deuterium.

Some isotopes, like deuterium, are stable, which means that they're perfectly happy with the number of electrons, protons, and neutrons they have. Other isotopes are unstable because the different number of neutrons interacts with the other atomic components in such a way that some of the bits go flying off and, over a period of time, the isotope changes into some other atom. When these unstable isotopes change to a different atom, they emit radioactivity. For that reason, they're called radioisotopes.

An important property of radioactive isotopes is that scientists can describe very accurately the average probability of the transition's happening and express that probability as a number called the half-life—the time it takes for half of the atoms to undergo this transition. In the first half-life, half of the atoms transition. In the second half-life, half of the remaining atoms transition, leaving one quarter of the original parent material. In the third half-life, half again transition, leaving one eighth, and so on. (Remember: Just because half the isotopes decay in the first half-life doesn't mean that the other half decay in the second half-life—you'd be surprised at the number of students who make this assumption. Only half decay every half-life.)

To determine the age of material, researchers compare the ratio of the parent and daughter products that were initially in the sample with the ratio of these products at the current time. By doing so, they can calculate how much time has passed. The atomic clock is a very accurate national timekeeping apparatus calibrated by the precise regularity of radioactive decay.

Numerous radioactive isotopes exist. One system that has been very successful in dating the ages of fossils is potassium-argon dating. Potassium is an extremely common element. Although most potassium isotopes aren't radioactive, one of them is, and one of its decay products is the gas argon.

Potassium-argon dating relies on the fact that although potassium is a solid, argon is a gas. When rock is melted (think lava), all the argon in the rock escapes, and when the rock solidifies again, only potassium is left. The melting of the rock and releasing of any argon present set the potassium-argon clock at zero. As time passes, argon accumulates in the rock as a result of radioactive potassium decay. When scientists analyze these rocks and compute the ratio of argon to potassium, they're able to determine how long it's been since the lava cooled. When scientists date rocks from our solar system this way, the oldest dates they find are 4.5 billion years.

No fossils are present in lava, obviously; anything that was there melted along with the rock. But by dating the lava flows above and below a fossil find, scientists can put exact boundaries on the maximum and minimum age of that fossil. In this case, the variation in possible ages of the fossil simply reflects the fact that the fossil exists between the dated lava flows.

Radioactive dating has been perfected to the extent that scientists can get within a few percentage points of the actual date. They know this because they're able to date lava flows that happened recently enough for their dates to be known historically. Potassium-argon dating has been used to date accurately the age of the eruption of Mount Vesuvius at Pompeii, for example. The scientists knew that the technique worked because the age their equipment indicated matched the age noted in historical Roman records.

Types of rocks and fossils

Geologists in Darwin's day were familiar with the diversity of types of rocks, but they were only beginning to appreciate the vast time scales over which geological events occurred. They had yet to understand the dynamic nature of the Earth's crust, and they lacked modern understanding of how these types of rocks formed:

I Igneous: Igneous rocks are of volcanic origin. They form when molten lava from volcanoes cools and solidifies. Basalt and obsidian are examples of igneous rocks.

I Sedimentary: Sedimentary rocks are formed by the gradual deposits of sediments. Sandstone is an example of sedimentary rock.

I Metamorphic: Metamorphic rocks are rocks of any origin that have been subjected to the extreme stresses and temperatures caused by the folding and crushing of the Earth's crust.

Understanding these rock types helps biologists understand the fossil record. Fossils are found only in sedimentary rocks. The molten lava that bubbles up from beneath the Earth's crust during a volcanic eruption doesn't contain any fossils (whatever had been there would have melted in the molten rock). Metamorphic rocks — even those of sedimentary-rock origin — don't contain fossils, because the extreme temperatures and pressures that converted the rock from sedimentary to metamorphic would have destroyed whatever fossils may have been there.

Today, scientists know quite a bit more. First, through radioactive dating, a painfully complex process whose details you don't need to worry about, they know that the Earth is about 4.5 billion years old. Scientists also know that life has existed on Earth for at least 3.5 billion years — a number that keeps changing as older and older fossils are found.

Although the age of the Earth may seem to be somewhat unrelated to evolution (rock, stone, and tectonic plates aren't living organisms and, therefore, don't "evolve"), it's actually very important to the theory of evolution because biological evolution needs time to happen. By knowing the actual age of the Earth and how long life has been present, scientists can ask whether enough time has passed for simple creatures such as the ones they see in the oldest rocks to evolve into more complex creatures, such as the ones that can write and edit books. The quick answer: Yes.

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