Carbon Dating: The History Of Life On Earth (Video)

How Old Is Life On Earth, And How Do We Know?

The Earth is 4.54 billion years old; 3.8 billion years ago, the very first life form came into existence; 225 million years ago, dinosaurs came on the scene; and man took his first steps in Africa 200,000 years ago. How do we know this to be true?


Carbon-14 and other radioisotopes are used to measure the age of fossils, rocks, and other materials that make up Earth's geologic history. These techniques are reliable and valid. They provide clues into our earliest origins. And they contribute to the hundreds of lines of evidence supporting Charles Darwin's theory of evolution, which continues to stand the test of time.

To learn more, watch the video above and read the transcript below. And don't forget to leave a comment at the bottom of the page. Talk nerdy to me!


Hi everyone. Cara Santa Maria, here. The Earth is 4.54 billion years old; 3.8 billion years ago, the very first life form came into existence; 225 million years ago, dinosaurs came on the scene; and man took his first steps in Africa 200,000 years ago. How do we know this to be true? Science.

One of the ways that researchers measure the age of organic material is through carbon-14 dating. In 1960, Willard Libby won a Nobel Prize for developing this technology. See, all living things contain carbon, which has six protons and six neutrons, so in its typical form, we call it carbon-12. But at any given time, there are trace amounts of carbon-14, or C14, in the atmosphere. C14 is a radioactive isotope that's made when cosmic rays bombard nitrogen atoms at high altitudes, converting them to this excited form.

When some living things, like plants and algae, make their own food through photosynthesis, they take in carbon dioxide from the air. Trace amounts of C14 make up a tiny percentage of that carbon dioxide, and it's integrated into the tissues of the organism. Then creatures that can't make their own food through photosynthesis (like us) eat the ones that can, and that C14 is taken into our bodies as well. And because there's a constant quantity of C14 in the atmosphere, there's a constant, corresponding quantity of it in the bodies of all living things, at least while they're still alive.

C14 is radioactive. That doesn't mean it's dangerous, only that it's unstable. Over time, it decays back into nitrogen. See, when an organism dies, it stops taking in carbon. And the C14 in the organism's tissues starts to decay at a precise speed, but the amount of carbon-12 stays the same, since it's not radioactive.

We know that it takes 5,730 years for half of the C14 in a sample to decay. It takes another 5,730 years for half of what's left to decay, and so on. This is C14's half-life. All radioactive isotopes have one. And if we compare the amount of C14 in a dead thing to the amount of regular carbon-12, voila! We can find out how old it is.

Now, some people who think that the earth is only 6,000 years old may base their claims on words in the Bible, not measurable evidence. And one ploy they use to cast doubt on radiocarbon dating is to point out its shortcomings. For example: C14 has a relatively short half-life. So, anything older than 50,000 years only has too little C14 left to make an accurate calculation of its age. But C14 isn't the only radioisotope out there. There are tons of them! If I wanted to find out the age of a dinosaur fossil, I might measure its uranium-235 concentration, which has a half-life of 704 million years. Radioactive isotopes like potassium-40 and rubidium-87 have half-lives in the billions of years.

Critics also like to point out that over time, the amount of C14 in the Earth's atmosphere may have varied. But scientists know this, so they make corresponding adjustments to their measurements. And radioisotope dating may be one of the more sophisticated methods we use to know the age of fossils, but it's not the only one.

Millions of fossils have been pulled from the earth. And by the 1800s, we realized that consistently and predictably, older rock is found below younger rock, and older fossils are found below younger ones within that rock. With age comes progress: younger things are more complex, more diversified.

When Charles Darwin published "On the Origin of Species" in 1859, he didn't have all the answers. We still don't. But I can tell you this for certain: evolution is a fact. Man never walked with dinosaurs and you and I are apes. Scientific advances are made every day in hundreds of disparate fields. And not one of these findings has served to undermine the theory of evolution. Not one. Not a single fossil discovery has been out of place, on the wrong continent or in the wrong rock. There's no complexity that's irreducible and no form that was intelligently designed. The data simply don't support it. And that's the greatest thing about evolution: it just is. And it'll keep on happening whether you believe in it or not.

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