A friend recently asked me if I knew why organisms get more complicated over time. After some thought, I realized that the premise of his question is actually false (or, perhaps more accurately, half true). It assumes that evolution is inherently progressive. However, natural selection is a blind process with no intrinsic direction, where organisms can be selected for either increased or decreased complexity in response to selection pressures. While it is true that complex organisms can be produced from simpler ones, the idea that the ultimate goal of evolution is the progression of simpler organisms into “higher-order organisms”, or that this is an inevitable byproduct of evolution, is a misconception. If it were true that evolution was inherently progressive, we would expect to see only eukaryotic (multicellular) organisms, roughly 3.5 billion years after the original “primordial ooze”. But in reality, extant life on earth is predominantly made up of archae and bacteria: simple, microscopic prokaryotes. Their respective biomass outweighs “higher order”, macroscopic eukaryotes 10 to 1. Large organisms only appear more diverse to us because of sampling bias. We notice them more because we can actually see them, whereas there are only a dozen or so unicellular species that are visible to the naked eye. We live in a macro-scale world, so we are naturally macro-scale chauvinists. Nevertheless, some organisms have reached a startling degree of complexity. Take the human body. On a microscopic level, it is a choreographed colony of trillions of cells. Zoom out, and you have complex structures like the eye. How and why did there get to be so much complexity?
The first thing that needs to be addressed is why we have bodies at all. What could inspire cells to abandon their freedom, banding together to form huge lumbering multicellular organisms like us? The answer is fairly clear cut: multicellularity allows organisms to exceed the normal size limits imposed by diffusion, conferring the competitive advantages of increased size. If you are larger, you have fewer predators and less competition. Another advantage of multicellularity, which has has nothing to do with size, is the ability to have differentiated cells.
Differentiated cells provide an advantage because they have the freedom to become highly specialized, skilled at performing a single task extremely well. However, as is the case with all adaptations, multicellularity comes at a cost. For instance, if you increase in size, your need for food increases proportionally. Also, only certain uni-cellular organisms are eligible for becoming multicellular. There are probably only a few, very specific ways of making the first, crucial step towards multicellularity. Thus, only under a strict set of conditions will multi-cellularity develop. With this in mind, it’s no wonder why not all prokaryotes have made the change. This goes to show how, although a potential adaptation may seem like an upgrade from our perspective, that doesn’t guarantee that it is going to happen. Evolution has no foresight, so an adaptation must be practical and advantageous from the get go.
The Origins of Multicellularity
There are three main hypotheses for how multicellularity evolved, with the third being the most credited by the scientific community.
The Symbiotic Theory
The symbiotic theory suggests that multicellularity resulted from a symbiotic relationship between different species of single-cell organisms, each fulfilling a different role. Eventually, they became so specialized in their respective roles that they were unable to survive without each other. This type of symbiosis can be observed in nature, such as in the relationship between coral and algae. In these cases, it is likely that if one species went extinct the other would follow. However, it is unclear how each organism’s DNA could have gotten combined into one genome, making them a true, single species. Therein lies the problem with this theory.
The Cellularization Theory
The Cellularization Theory states that a single unicellular life form developed internal membranes around each of its multiple nuclei. Over time, the separated nuclei differentiated in their functions, resulting in a multicellular creature similar to modern turbellarian flatworms. However, evidence from molecular and morphological data has demonstrated that this is probably incorrect.
The Colonial Theory
Proposed by Ernst Haeckel in 1874, the Colonial Theory claims that the symbiosis of multiple members of the same species (as opposed to separate species like in the Symbiotic Theory) culminated in the first multicellular organism. The advantage of this theory is that this behaviour has been observed in several species of protista. For example, the amoeba Dictyostelium has been known to group into colonies, moving on to new locations in search of food. Some of the amoeba then differentiate from each other, a necessary precursor of multicellularity.
Regardless of the mechanism by which it develops, multicellularity, once it is achieved, stimulates further increases in complexity. This is because cells suddenly have a completely new environment, where they differentiate into numerous lineages which cooperate within the organism to work toward shared goals. From a purely evolutionary perspective, propagation is the only real goal. However, there is a tall order of tasks that must be undertaken to lead up to propagation and ensure that it is possible: maintenance of homeostasis as a stable whole, response to stimuli, growth and development, and finally, reproduction. Any mutation that aids in the completion of any one of these tasks has a good chance of staying in the gene pool, and therefor furthering the evolution of the organism–likely resulting in increased complexity.
Behold, the human eye. It refracts and focuses light, and then transduces the raw information into nerve signals, which are fed into the brain via the optic nerve to be filtered and patched together into something that we can make sense of. What could stimulate the evolution of such a complicated piece of optical machinery? How could it have evolved, when at first glance it seems like all the components would have to be there from the beginning for it to work at all? Darwin was troubled by this, and those opposed to evolution are understandably fond of quoting the following passage from On the Origin of Species:
“To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.”
However, they conveniently neglect to mention the remainder of the section, where he says that he has no problem believing that such a structure could have evolved:
“…If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.”
Evolution of the Eye
First of all, the evolution of the eye was a step-by-step, cumulative process. It didn’t just spring into existence fully formed through some astronomically lucky macro-mutation. That’s impossible. Things like that don’t happen by random chance, even over billions of years. Only with the help of a non-random process like natural selection can something like the eye come into being. Sadly, we can never know exactly how it happened, as the fossil record isn’t a perfect catalogue of intermediary forms, and even if it was, eyes are composed of soft tissues and don’t readily fossilize. Fortunately though, there are various creatures alive today with proto-eyes. Because they represent different stages in a process that could lead to an eye like ours, we can use them to assemble a hypothetical evolutionary pathway that could have been taken to arrive at the eyes we have today.
Half an eye is better than no eye, and even one percent of an eye is better than no vision at all. Even a mutation resulting in a single photoreceptor cell would confer a clear survival advantage, especially in a prehistoric world where blindness is the norm. Starting from that first photoreceptor cell, random mutations resulting in slight improvements in image fidelity provided survival advantages. These accumulated over time, resulting in a steadily increasing ramp of complexity.
Here’s the leading model used by evolutionary biologists to elaborate how this could have happened:
A mutation resulted in a single photoreceptor cell, which helped to calibrate circadian rhythms by detecting daylight. Over successive generations, possessing multiple photoreceptors became the norm in the gene pool, because individuals with mutations encoding for an increased number of photoreceptors were better able to react to their surroundings. An arms race began, fueling the evolution of the new sensory organ. Eventually, what was once just a single photoreceptor cell became a light-sensitive patch. At this point, the creature was still only able to distinguish light from dark. A slight depression in the patch created a pit, for the first time allowing a limited ability to sense from which direction light or shadow was coming from. The pit’s opening gradually narrowed to create an aperture, like that of a pinhole camera, making vision sharper. A transparent tissue formed at the front, with a concave curvature for refracting light, drastically improving image fidelity. And here we have the basic blueprint for a human eye. Every change in this process, however small, provided an advantage. It was a logical progression from 1% of an eye to 100% of an eye, and each stage was useful to its possessor.
As I mentioned before, organisms with proto-eyes corresponding to every step in the evolutionary sequence have been found. With the right selection pressures, they just might be navigating their environments with eyes like ours one day. Here are some examples.
1. Uni-cellular protists of the genus Euglena posses a small stigma, or eye spot, that is capable of detecting light, but unable to form images.
(© CC 2011 Deuterostome)
2. Planarian worms have cup shaped eyes, capable of detecting the direction of a light source.
4. Sea snails have a rudimentary lens in the form of a blob of jelly, giving them the ability to adjust their focus.
(© CC- 2007 Steve Childs, http://www.flickr.com/people/steve_childs)
Darwin recognized that the idea of design arising in the absence of a designer defies intuitive common sense. In a later edition of On the Origin of Species, after he had had a chance to witness the publics reaction to his theory, he stated his opinion that their incredulity was just a failure of imagination:
“When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of vox populi, vox dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory.”
Indeed, science has demonstrated over and over that the truth of things often defies common sense. To understand why we have such a hard time swallowing concepts like evolution, I think it is helpful to look at things through the lens of psychology. As a species, we are hardwired to ascribe intention and agency in an attempt to predict the behaviour of other beings. However, we are so hyper active in our propensity towards agency detection that we often ascribe agency to inanimate objects and natural forces. One byproduct of all this is a tendency to assume that anything that appears complex and purposeful must have been designed by an intelligent agent. This is exemplified by the Teleological argument, which William Paley described using his watch maker analogy. He argued that in the same way a watch’s complexity implies a designer, the evident complexity of nature implies the existence of god. In Paley’s day, it made more sense to make this argument, as a mechanism for how life could have evolved by itself had yet to be proposed. Today, in the post-Darwin days, the people making it really should know better. The theory of evolution, like the theory of gravity, provides a huge predictive capacity; it is a rigorous testable model of an observable phenomenon. We know that it is true, beyond all reasonable doubt, through a convergence of evidence from fields such as: palaeontology, geology, botany, zoology, comparative anatomy, bio geography, and genetics. But still there remains a barrier of doubt that, for many, all the evidence in the world won’t surmount. To grasp the rather counter-intuitive fact that “design” can spontaneously emerge in the absence of a designer, I recommend checking out Conway’s Game Of Life, a mesmerizing cellular grid program that models how complex patterns can emerge from the implementation of a few simple rules. As you watch the spiralling patterns and geometric shapes generate from practically nothing, I guarantee that your common sense will be highly offended.