The Earth is 4.54 billion years old; 3.8 billion years ago, the first life forms appeared; 225 million years ago, dinosaurs came on the scene and man took his first steps in Africa about 200,000 years ago. How do we know this?
One of the ways that researchers measure the age of organic material is through carbon-14 dating. In 1960, Willard Libby won the Nobel Prize for developing this technology. All living things contain carbon, which normally 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 in the atmosphere. Carbon-14 is a radioactive isotope that’s made when cosmic rays bombard nitrogen atoms at high altitudes, converting them to this more 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 Carbon-14 make up a small 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 carbon-14 is taken into our bodies as well. And because there’s a constant quantity of carbon-14 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.
Carbon-14 is radioactive which doesn’t necessarily mean it’s dangerous, only that it’s unstable. Over time, it decays back into nitrogen. When an organism dies, it stops taking in carbon. And the carbon-14 in the organism’s tissues starts to decay at a precise speed, but the amount of carbon-12 stays the same, since it’s stable and not radioactive.
We know that it takes 5,730 years for half of the carbon-14 in a sample to decay. It takes another 5,730 years for half of what’s left to decay, and so on. This is carbon-14′s half-life. All radioactive isotopes have one. And if we compare the amount of carbon-14 in a dead thing to the amount of regular carbon-12, we can find out how old it is.
Some people who think that the earth is only 6,000 years old base their claims on interpretations of the Bible, not measurable evidence. And one ploy they use to cast doubt on radiocarbon dating is to point out its shortcomings. For example: Carbon-14 has a relatively short half-life. So, anything older than 50,000 years old has too little carbon-14 left to make an accurate calculation of its age. But carbon-14 isn’t the only radioisotope. There are dozens of them. If I wanted to find out the age of a dinosaur fossil for example, 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 carbon-14 in the Earth’s atmosphere may have varied. But scientists, generally being very intelligent people, already know this, so they make corresponding adjustments to their measurements. And radioisotope dating is 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: more recent life forms are more complex and 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 ever 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 does not support it. And that’s the great thing about evolution: it just is.
And it’s true whether you like it or not.