by James Morris
There are some phrases that capture the essence of an idea in a way that a lengthy discussion does not. These are ones I remember and even use in my teaching, hoping they will stick with my students too. Many of these come from the some of best science writers we have today.
Here are a few:
We inhale oxygen and exhale carbon dioxide. Yet neither oxygen nor carbon dioxide is the most abundant gas in the air we breathe. Oxygen makes up just 21% of air by volume, and carbon dioxide only accounts for just a tiny fraction of a percent. Most of the air we breathe is nitrogen gas (N2), which makes up 78% of the air in our atmosphere.
Nitrogen is essential to life. It makes up proteins, like hemoglobin, and nucleic acids, like DNA. So we need it. But we can’t use the nitrogen that is literally all around us. The nitrogen in the air is in a form that is not accessible to most organisms, including humans.
Some microorganisms, like bacteria, are able to convert nitrogen gas into a form that can be used by other organisms. Scientists use the term “fixed” to describe nitrogen that is biologically available. So, bacteria can fix nitrogen in the air to forms that we and other organisms can use.
That’s a lengthy explanation. In a recent issue of The New Yorker, staff writer Elizabeth Kolbert puts it much more succinctly: “Nitrogen is a tease,” she writes. I love that. It gets right to the heart of the idea – nitrogen taunts us by being plentiful but unavailable. That’s what I remember.
Here’s another one that I really like:
It’s hard to define life. We all recognize it, but when pressed to come up with a definition, we run into problems. Reproduction, for example, is a hallmark of life. However, computer viruses replicate but are not alive, and mules don’t reproduce but are alive.
Life can’t be defined by one attribute, but several. To get at this idea, Natalie Angier, writing in The New York Times, described three essential aspects of a cell, the smallest unit that is considered living. These are a stable archive of information (DNA), a membrane or boundary to separate inside from out (the cell membrane), and the ability to harness energy from the environment (from the sun or chemical compounds). That’s it. If you have these three things, you have a cell. And if you have a cell, you have life.
But she didn’t call them essential aspects of a cell, as I just did. Instead, she called them “the three cellular non-negotiables.” That’s perfect. It really captures the idea that these are indeed essential and not to be messed with. There’s no deal, or life, without them.
Another concept that’s hard to define is “species.” Like life, we know a species when we see one. Most of us think that a species includes organisms that look alike, and different species look different. But it turns out that that’s not always the case.
Consider the colorful red male Northern Cardinal and the much more drab, brownish female Northern Cardinal. They actually look quite different from each other, but are members of the same species. And there are plenty of different species that are impossible to distinguish from each other by sight alone.
The problem here is that species change over time. So, we are trying to define something that is fluid. John Whitfield, in his “Blogging the Origin” gets right to the heart of this difficulty, not with a word or phrase, but with a memorable sentence. He writes, “If nature does not fit into our boxes, the task is not to build a better box, it’s to work out what that uncertainty is telling us.”
In other words, don’t spend time trying to come up with an air-tight definition of “species” that can be applied in all cases, but use the fact that a species is hard to define to better understand what a species is.
One of the ways that species change is by incorporating other organisms. This tight partnership, or endosymbiosis, accounts for some of the dramatic leaps in evolution. Some plants, for example, incorporate nitrogen-fixing bacteria in their roots so they have a steady supply of fixed (readily available) nitrogen.
Endosymbiosis also accounts for the spread of photosynthesis through many different species. The first organisms capable of photosynthesis were cyanobacteria. Cyanobacteria use the energy of sunlight to produce carbohydrates. Photosynthesis later spread to eukaryotes – organisms with cells that have a nucleus – when a cell engulfed one of these cyanobacteria, and instead of digesting it, incorporated it, becoming in time a chloroplast.
Photosynthesis then spread to other branches of the eukaryote tree, not by reinventing the biochemical machinery of photosynthesis, but instead when other eukaryotes engulfed and incorporated photosynthetic eukaryotes.
Again, a bit complicated. But Carl Zimmer has an image that can help us. He describes it as a “Russian-doll sequence of events.” Everyone is familiar with these kinds of dolls, the ones where one fits snuggly into the next. That’s a great image for helping us learn and remember an otherwise confusing and complex sequence of evolutionary events.
Evolutionary innovations occur in lots of different ways, not just by endosymbiosis. One common way is to take a structure originally used for one function and repurpose it for a completely different function. Our jaws evolved from structures that were originally used to support gills. And our tiny inner ear bones were once part of the hinge of the jaw.
The late evolutionary biologist Stephen Jay Gould has lots of turns of phrases that are hard to forget. His columns that appeared regularly in Natural History are filled with them. But here I will point out a title of one of his famous collection of essays, The Panda’s Thumb. This title is a wonderful way to describe what I just mentioned, co-opting one tool for another purpose.
What he was describing was what looks like a bona fide thumb, used by pandas to grasp bamboo. But, it turns out, the panda’s thumb is not really a thumb at all – it’s a modified wrist bone, as Gould explains.
Gould’s columns were called “This View of Life.” That phrase itself was borrowed from none other than Charles Darwin. Darwin concludes On the Origin of Species with arguably one of the most well known sentences in any piece of scientific writing. In this last sentence, he talks about the wonder of nature, compares his theory of evolution by natural selection with Newton’s law of gravity, suggests the possibility of a single origin for all of life while also leaving room for a Creator, and ends with an emphatic nod to evolution.
I will therefore leave the last words to Darwin:
“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
© James Morris and Science Whys, 2016.