(This is part 2 of 4 in a series dedicated to the concept of "junk DNA". Part 1 is here.)
Carl Sagan used this beautiful video to illustrate evolution:
(And Vangelis's soundtrack is the cherry on top!)
You might think that the same happens to DNA: one mutation after the other, DNA branches out just like the organisms in the video. That is not quite the case. Most of the information is saved, not erased. Why? Because that is the smart thing to do.
Mutations typically occur as random errors when cells duplicate. This often results in a new, non-functional gene. The old gene is still functioning, and the new one has a mutation that may or may not be deleterious. As mutations accumulate, things shift. The new gene may end up being functional, and, if the new mutation doesn't alter the information (what we would call a "silent" mutation), it will perform the exact same function as the old gene. That's what we call "redundancy," in other words, two or more genes sharing the same functionality. This is advantageous because if one suddenly loses its functionality, the system can revert to the old one to restore the information. It's the same mechanism used in CDs and DVDs, for example, so that you can hand them off to your kids and, unless they decide to use them as frisbees, a few scratches won't ruin your music or favorite movies.
What if the mutation was not silent and it did change the functionality of the gene?
First of all, it takes many mutations and many generations for this to happen. Sagan's video summarizes million of years in just a few minutes. But when it does happen, the new gene takes over and becomes functional. And the old gene? Still there, stored away. If you compare genes to switches, whenever a "new" gene arises, the old one is turned off and a new switch is made and turned on.
We all share most of our DNA with monkeys, giraffes, elephants, mice. What changes is which genes are "on" and which are "off." The non-functional genes that we share with other organisms are called "pseudogenes," and they are non-coding. What this means is that when DNA is unfolded and prepared for the retrieval of information to make the proteins that keep us alive, all those pseudogenes are tossed away. And that's what led to the term "junk DNA." But you see, pseudogenes have a very important role in evolution.
Imagine to toss a ball onto a rugged landscape. There are infinitely many paths the ball can take. Evolution is a rugged landscape and organisms are balls competing for the equilibrium niches. In an ever-changing landscape, saving the information can be vital. How did mammals end up back in the ocean? Because it had become advantageous and the information was already there. A few pseudogenes became functional again and the switch happened.
Again, these changes don't happen overnight. We are talking about hundreds of generations. And there is no intention behind the changes, only pure randomness driven by selection pressure from the environment. We are changing our planet a little too fast for evolution to save us. But if we had enough time, eventually our body would adapt to a CO2 laden atmosphere as the first micro-organisms that populated the Earth.
That's because Mother Nature took care of everything.
Picture: Mirror installation at the Metropolitan Museum of Art, New York. Canon 40D, focal length 66mm, exposure time 0.3 seconds.