I WANT YOU TO LOOK AT YOUR HANDS. Go ahead, do it right now and observe every structure you can. Take note of the placement of the bones and muscles, the tendons and ligaments, and the interface between all of them. Notice how they all fit together gracefully and move with awesome precision. Now think about why you have them, and consider the billions of years leading up to their evolution. Think about all the events that decided you would have five fingers with hard pointy structures at the tips. And now imagine a whale skeleton. It bears some similarity to the human one, like the rib cage and five sets of “finger” bones. It even has a pelvis, albeit a highly reduced one that assumes no role like it does in humans.
Keeping with that comparison, look at a human pelvis’ coccyx, or tailbone. It doesn’t seem to serve much of a purpose in our body plan; it’s just another thing you can break. But it extends into a tail in many other mammals. These anatomical similarities, differences, and oddities are humbling. They unify us with so many other creatures and should be a reminder of how intertwined our story is with theirs. Our species matured alongside the other mammals and we are literally like a family. At first glance the differences between a human and an elephant might be striking, but if you look at their muscles or skeletons or circulatory systems you will see recurring themes as though we were variants of the same template. And if a comparison of major structures hasn’t convinced you how related we are, our similarities on the cellular level are truly undeniable.
Something that has always fascinated me is speculating how different the story of life on Earth could have been. Could human beings have evolved with horns and a tail, for example? Before we can tackle this provocative question, it’s wise to understand the unit of life – the cell – and why it is the way it is. As before, vast morphological changes do not require vast changes in genetic identity. The observable differences between two mammals of your choice are superficial. Think about it this way: we have mostly the same organs as other mammals. If we have mostly the same organs, our tissues are similar. That means our cells are similar, and so on to the level of biomolecules themselves.
Even more unifying is our dependence on oxygen, which is shared not only by other mammals, but by insects and many unicellular organisms too. Further, the submicroscopic machinery that we (all eukaryotes) possess to utilize oxygen in our energy-generating processes is identical. Basically, fats, carbohydrates, and proteins can be fed directly or indirectly into a metabolic pathway known as the Krebs Cycle, which produces high-energy molecules (NADH) full of electrons that are ready to fly off. NADHs are stripped of their electrons, which are then shuttled along an array of proteins embedded in a membrane within the cell. At each protein, the energy carried by the electrons is used to build a gradient of hydrogen ions (H+) across the membrane. Eventually, this gradient’s energy can be used by the ATP synthase, a micro-machine that spins like a turbine as H+ ions fire through it to build energy-rich molecules called ATP. One single ATP synthase produces hundreds of ATPs per second, and in total, your body makes about 1021 (one sextillion) ATPs per day. ATP is the versatile “energy currency” of the cell, used for virtually every energy-requiring process. And ATP is pretty much shared by all organisms. All of them. So forget skeletons and muscles; ATP is universal.
And here is where oxygen comes in: the electrons eventually combine with the O2 you breathe in and a few H+ ions, making water. In other words, oxygen, an explosive gas capable of turning iron into rusty scrap metal, is the substance that permits life as you know it to exist and grow. Without it, we’d likely have never made it past the unicellular stage.
So to answer the question about life starting over, it’s safe to assume that rudimentary biomolecules would assemble the same way given the same primeval conditions. There isn’t much wiggle room for building molecules that can self-replicate and catalyze reactions, especially when one is starting with the same compounds believed to be present in the primeval atmosphere: water, methane, ammonia, hydrogen, etc. These molecules would then be enclosed in a membrane, whose properties are dictated by physical laws, if any progress is to be made. In essence, the cell would be the fundamental unit of life because no other construct we can think of can assume the same role. Oxygen would play a big part once it found its way into the atmosphere, and organisms that exploit its chemistry for metabolic processes would rise above their counterparts. There would be a need for metabolic catalysis, so diverse enzymes would appear. And there would be a need for a molecule that encodes instructions to fabricate proteins, so DNA or some exotic relative would be adapted to this role. Multicellular organisms would be only a step away, and we would find our way onto land again. At that point, it’s anyone’s guess what we would look like.
What if life didn’t start all over again, but time was kind of “rewound” to, let’s say, the Jurassic Period? Many of our organ systems (never mind cells and proteins!) would already be present in the little creatures scurrying about. Since the evolution of our macroscopic appearance is so chaotic, maybe we would have ended up looking more like furry critters with pointy ears. Maybe some minor event would have tipped the scales against mammals, allowing reptiles to emerge as the dominant species and eventually become the “humans” of that timeline. This is only speculation, but it should inspire awe toward our present appearance and the features we’ve inherited. After all, the Homo sapiens you know might have been the fantastical fabrication of some lizard man’s mind. And despite that appalling possibility, however foreign it may seem, the cell and its biochemistry – the engine of life – would exist and function almost exactly as we know it.
Alex Cojocaru is a fourth-year student at the University of Alberta, entering his first year in Medicine.