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Rock of Ages

Posted: July 29, 2014 5:29 p.m.
Updated: July 30, 2014 5:00 a.m.

I remember once I was giving a presentation about important conservation properties in the Piedmont. I showed photos of the incredible rock formations on a particular property and happened to mention their age in an effort to describe their grandeur. Afterwards, I was confronted by an indignant man who told me that the age of rocks cannot be known. He accused me of making those figures up out of thin air. Surprised by his vociferous tone, I told him I was sorry to have upset him. While not a confrontational person, I am a teacher, and I began to politely explain how the ages of rocks are discerned.

When rocks crystalize out of magma, certain radioactive elements are incorporated into them. Other elements are found outside of rocks. These elements decay over time at constant rates that are well established. You may have heard of a “half life,” which is the way this time period is expressed. Carbon gives us estimates of sixty thousand years or so, while uranium can date back billions of years. While true there is a margin of error in this estimation, this margin is relatively small compared to the total number of years. For example, Uranium may be off by two million years, but that isn’t bad when estimating rocks two billion years old.

This brings us to another piece of evidence in our series on evolution. That is, when traces of plants and animals are imbedded within or near such elements, we can get good estimates of their age too. We call these traces fossils, and they come in the form of bones or prints that remain long after the life of an organism has ended. In the last column, we defined evolution as saying that life originated from an original life form that branched out gradually over billions of years, producing the diverse life forms we see today. If so, we might expect to observe a couple of phenomena in our fossils. We can put these expectations into a hypothesis and say that in our exploration, we will find that older organisms are simple, while younger ones are more complex. Moreover, we can predict that older fossils will be very different from living species, while younger fossils will be more similar. Indeed, both of these patterns have been found by thousands of studies addressing this hypothesis. And in those studies, not one fossil has ever been found out of sync with this predicted chronology. If evolution were not true, surely by now we would have found some exceptions.

Another piece of evidence often discussed in biology books is that of the “vestigial trait.” This is a feature of a species that was an adaptation in its ancestors but that has either lost its usefulness or been usurped for a new use. For example, whales have hind limbs and pelvic bones like other mammals. Oddly, these bones are completely detached from their skeletons and simply imbedded in tissue, providing no function. This makes little sense unless viewed through the lens of evolution, which says that whales evolved from a common ancestor who dwelled upon land and used these bones for walking. Indeed, fossil evidence like that already discussed also supports this claim.

In humans, the appendix is among the most obvious of our many vestigial traits. Evidence suggests this organ was once used to help us digest cellulose. Other leaf-eating primates have a larger appendix than ours. In them, it hosts bacteria to help digest cellulose. As expected, primates who eat fewer leaves have a smaller appendix. We don’t eat many leaves and we can’t digest cellulose. Therefore, we have a small appendix, and some folks are even born without one. While it may have some unknown function, that function is not clear. So why do we have an appendix? We have an appendix because our ancestors had an appendix. Like the bones inside the whale, it is strange we would have an appendix if evolution were not a real and active process in our lives.

I don’t think any of this ever resonated with the gentleman who confronted me after my presentation. He continued to bombard me with literature saying that the earth was of a much younger age than evidence supports. Despite our differences and our less than amiable introduction to one another, we developed a lasting friendship centered on golf, of all things. He was an elderly man, but his shots were straight and accurate. He enjoyed a silent satisfaction in stomping me every time we played the game. As with so many in our older generation, I loved hearing and learning from his life stories. Whenever evolution came up, he’d just tell me I was horribly misled, and in time I would say the same right back to him. We managed to laugh about our differences, and looking back, I think that is what I enjoyed most about the time I shared with my friend.

(Jenkins teaches Natural History of South Carolina at USC Sumter and maintains a nature blog at


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