by James Morris
November 24, 2014 marks the 155th anniversary of the publication of Darwin’s On the Origin of Species. Anniversaries are an opportunity to look back – in this case, way back. The Origin changed the way we look at the world and gave us a new window on the past. This essay is a celebration of our remarkable history.
In golf, we are told to focus on the next shot, to just look ahead. Don’t worry about past mistakes: a shot in the bunker, a slice that spins the ball out of bounds, an errant drive that enters the water with a loud and emphatic splash. Perhaps this is why many see golf as a lesson for life: Don’t dwell on the past. Move on. No “what if’s” or “if only’s.”
In biology, however, there is no such forgetting the past. We carry our history with us. It’s literally in our DNA, in the very structure of our bodies. You don’t have to look far for evidence of this fact. It’s clear when we look at our bodies through this lens.
Our windpipe and esophagus cross, so that food sometimes “goes down the wrong pipe.” Why is this?
Most immediately, it’s because they develop that way. Our windpipe, in fact our entire respiratory system including the lungs, is an outgrowth of our gut. As an adult, we simply retain the connection between the two.
Taking a broader view, that’s the way swimbladders develop in fish. Swimbladders, used for flotation in fish, and lungs, used for breathing in humans, are not just similar in form and anatomical position, but evolved from the same structure in the common ancestor of modern-day fish and humans. In this common ancestor, that’s the way it was, and, simply put, we are stuck with it.
It’s kind of like the keys on a keyboard. Look at the six letters in the upper left: Q-W-E-R-T-Y. This is why it’s called a QWERTY keyboard. Why is it organized in this odd fashion? One idea is that it goes back to the days of the typewriter. A problem with typewriters is that the letters at the end of the arms sometimes get stuck, especially with fast typists, so the designers of the typewriter needed a way to slow things down. The QWERTY keyboard does just that. It purposely slows down fast typists so that the letters don’t get all tangled together.
Why, then, when we switched to computers with no mechanical arms, did we keep this arrangement of letters? The answer is that we were used to it. As described recently in The New York Times Magazine, a more intuitive arrangement, the Dvorak Simplified Keyboard, never took hold, teaching us the power of history in shaping our habits.
History can also constrain evolution. Evolution does not work through forethought or with the goal of building a respiratory tract (or anything else for that matter). It works by taking what it has and working with it. Like a tinkerer, as the evolutionary biologist Stephen Jay Gould put it.
We can find signatures of our long history no matter where we look. Inside our cells, we see evidence of past interactions. Our very cells are the descendants of an early, intimate partnership between an ancestral cell and a bacterium. The bacteria have been reduced to small membrane-bound compartments inside our cells, known as mitochondria, which play a key role in energy metabolism.
Going back even further in time, we retain features that reflect where life originated. It is thought that life originated in water, and life evolved in a watery environment during the first couple of billion years of life on Earth.
We, and all organisms, “remember” where we came from. Water is essential for life. We are something like 70% water. Although we can go several weeks without food, we can only go a couple of days without water. We still develop in a watery environment – the amniotic fluid in which fetal development occurs.
Water is also the main ingredient of our blood, transferring oxygen, nutrients, and cells of the immune system throughout our body, while at the same time helping us to remove wastes. All of the chemical reactions that take place in our body – all of them – take place in an aqueous (watery) environment.
All organisms, in fact, are mostly water, and require water to stay alive. No organism can reproduce without it. Water is truly life-giving and life-sustaining.
Even before life originated, it is thought that one particular molecule, called RNA, might have been the first, or at least one of the first, key molecules in the evolution of life.
Even today, we see RNA in many essential molecular processes: it’s the go-between from DNA (where genetic information is stored) and proteins (which carry out much of the work of the cell). DNA requires a small stretch of RNA (a primer) to make copies of itself. RNA is also part of ribosomes where proteins are synthesized.
Small RNAs called tRNAs do the actual translating between the nucleic acid code of DNA and the amino acid code of proteins. Recently, additional small RNAs that don’t code for proteins are turning out to be widespread in the human and many other genomes.
In other words, RNA crops up all over the place, in essential processes of life, perhaps a remnant of the time when it carried out many of these functions on its own.
We not only see our past by looking at what we retain, but also in structures that were once useful but now are less useful or no longer useful at all. Charles Darwin, in On the Origin of Species, doesn’t miss the opportunity to point out how rudimentary structures, as he calls these vestigial organs, are one of the arguments for evolution: “The meaning of rudimentary organs is often quite unmistakeable,” he writes.
Our appendix, Darwin points out, is the vestige of the cecum, once used to house bacteria useful for digesting plant matter. When our diet changed from one that was more plant-based to one that was more meat-based, the cecum, over time, became smaller and smaller, until today it is quite literally an appendix of our gut.
Other vestigial structures – like the remnants of hipbones in whales and wings of flightless birds – are striking windows into the past.
As Gerald Joyce of the Scripps Research Institute once put it, “Biology…actually records the history. Like a palimpsest, a document that’s written on again and again and again, biology is a record of what’s proven adaptive at the time, time after time after time.”
© James Morris and Science Whys, 2014.