How plausible are Humanoid Aliens as often depicted in science fiction? The previous part of this video series featured a global introduction to this question. In this part, we are going to take a closer look at the general human body plan. What major elements is it made of and how or why did each of these evolve? More importantly, how dispensable are these different parts and does the animal kingdom offer clues as to any possible alternatives? Is the human body plan an unavoidable epitome of evolution or can we expect sentient, tech-savvy species on other planets to look completely different? Let’s find out!

Let’s pretend that this is the first time we ever saw a human being and tried to make heads or tails of their body. The first thing that becomes obvious is that it can be divided into several main parts: There’s a distinct head, connected to a torso through a neck, and two pairs of limbs: the arms and legs. The torso itself can be roughly divided into the chest or thorax, the abdomen and the pelvic region. Each division holds specific organs and can more or less be said to have its own general function or set of functions.

Gross model of the human body plan.

The head contains the brain, the main sensory organs like the eyes and ears, and the mouth. The mouth is of course the entrance point for both our digestive tract and our breathing apparatus, the latter together with the nose, through which we also use our sense of smell. The chest or thorax is a sturdy box protecting the vital organs of the heart and lungs, making it the central hub of the circulation and respiratory systems. It is also where our first pair of limbs are attached: The arms. The abdomen holds a body cavity containing the stomach, intestines and other organs related to digestion and waste extraction, processing nutrients and waste. The pelvic region contains the bony pelvis, which is there for to provide joints and muscle attachments for our legs, so it could be regarded as an extension of the latter. The pelvic cavity contains the reproductive organs, urinary bladder as well as the final end of our digestive tract, making it the endpoint for several important bodily processes.

But is there any reason that these body divisions have to be arranged in this particular order or that each division should contain the particular groups of organs that they do in humans? Let’s look at some other highly evolved groups. Arthropods like insects, spiders and lobsters for instance have somewhat different body divisions.

Simplified models of different invertebrate body plans: Insects (Hexapoda), Spiders (Arachnida) and Squid (Cephalopoda).

Insect bodies have a seemingly similar arrangement called head, thorax and abdomen. Again, the head holds the major brain, sensory organs and mouth and is connected to the rest of the body through a neck. Besides that, the insect thorax holds the stomach and the abdomen holds major parts of the circulatory and respiratory systems. So the terms “thorax” and “abdomen” are a bit misleading as they’re not exactly the same thing. The insect thorax also holds all limbs: The three pairs of legs, as well as the wings, and there is therefore not really a pelvis.

Spiders, scorpions and lobsters have comparable arrangements, but here the head and thorax are usually fused into what is called the cephalothorax or prosoma. There are different kinds of respiratory systems in these groups that vary as to their position. Cephalopods like squid and octopuses have a completely different arrangement all together! Here the head is placed in the middle with limbs on one end and the entire rest of the body on the other.

So for evolution to arrive at a certain body plan for a creature seems to be a case of mix & match. We could easily come up with a whole range of alternative arrangements from these basic building blocks to get some notion of how unremarkable the human body plan is. We could also conceive of body divisions that are unseen in Earth’s biological diversity and many of the divisions mentioned could be left out or divided further still. However, the layout of body functions isn’t completely random. There are some evolutionary reasons why we keep on seeing certain recurring patterns. For starters, the organ systems contained in the head often occur in close association.

But what prompted our distant ancestors develop a head in the first place? Well, that all depends on how an animal feeds and what that means for its symmetry. An animal that feeds passively by being sessile can have an almost random shape with little to no symmetry whatsoever. Like a sponge for instance. Sponges lack tissues and organs, however, so to get a more complex organism, you’d need to have more control over how the body is laid out. First of all, you’d need cells to stick together in distinct tissues. Secondly, for any kind of symmetry, you’d need a way to define sides, like for instance how a magnet has a positive and a negative pole, so some kind of polarity giving an axis. And thirdly, once a polarity has been established, you’d want some kind of markers to indicate what organs are to appear where. And this automatically opens up for repetition.

A good example is a creature like a polyp that has two definite sides and thus polarity. It has the mouth on one side and and a foot for attaching to a substrate on the other. Around its mouth is where tentacles form at regular distances making for a special kind of symmetry: radial symmetry. This is the basic architecture of Cnidarians with the body laid out around a central axis. Sedentary animals like anemones and floating ones like jellyfish don’t have a particular direction to go to in order to get food, so radial symmetry works for them. The evolution of the head is ultimately the result of an evolutionary innovation to make one end the leading edge of the body and the other end trailing behind it. Basically, it’s about adding a second polarity or axis to the animal body. This time between front and end. Combined with top to bottom polarity, it gives the animal two mirror image sides. This is called bilateral symmetry. It has perhaps originated only once during animal evolution on Earth, but it was a stunning success and led to a wide variety of different creatures, collectively called bilaterians.

In the wake of the adoption of bilateral symmetry, a specialisation took place of that end of the body that always comes into contact with the substrate first. Probably because it’s beneficial to be able to react to both bad and good things as fast as possible, most of the sensory organs started to appear at that end. With that came a concentration of nervous tissue to quickly process the sensory input, in other words: the beginnings of a centralised brain.

This is an evolutionary phenomenon called cephalization Certain obscure flatworm-like creatures grouped together as the Xenacoelomorpha appear to be the simplest example of this kind of body organisation. Not having a definite mouth, their gut has only one opening, which is on their belly side. With their head they seek out food to move their body over and ingest with their belly. The next innovation for bilaterians was making food enter in one end of the gut and exit the other end leading to a mouth and an anus. The consequence of bilateral symmetry on the one hand, and a directional gut on the other, is that you can expect certain bodily functions to occur in the same positional sequence. With the sensory organs, brain and mouth in the front, you can expect a kind of stomach for storing and preprocessing food first, followed by an intestine for extracting the nutrients. And this is the basic configuration we see for most bilateral creatures.

On the other hand, we shouldn’t underestimate the quirkiness of evolution and its capacity to pass up seemingly obvious and optimal solutions. Squid and other cephalopods usually swim in the opposite direction of what they’re “supposed to”, with the mouths and legs trailing behind the part of the body with the other organs. And echinoderms like sea stars and sea urchins evolved from bilateral ancestors but decided to opt for secondary radial symmetry after all. These animals can basically decide which part of the body to move forward based on their mood and external stimuli. They have no definite front end and therefore no centralized brain.

But even if we took bilateral symmetry for granted as a likely outcome of any evolution under Earth-like conditions, that still leaves us with a plethora of options beyond those leading to the human body architecture. To begin with, why do we even have a neck and do we really need it? The neck supports the head, enabling it to move independently from the rest of the body. This way, a creature can quickly look around and also move the mouth closer to a food source without having to move the entire body.

But there are other ways to solve this without a loose head. In decapod crustaceans, like crabs, and arachnids, like spiders, the head and the thorax are in one piece, as noted earlier. The lack of a neck is solved by having eyes on all sides of the head like in spiders, or eyes on stalks like crabs. Many of these creatures also have appendages like claws that can be used to move food to the mouth. So the neck is by far a necessarily expected outcome of evolution. And even the mouth doesn’t necessarily need to be on the head. It could also be at the end of a trunk or similar as long as it can be moved towards food. Extinct, primeval creatures like Opabinia seem to hint at that solution.

And there is the issue of limbs like arms, legs and other appendages that can vary widely in number and placement among different creatures. Fewer arms and fewer legs may seem less costly and therefore more optimal. But more legs enable faster movement and more arms would allow for handling more food. So what really is optimal? Remember that there are many complex creatures with bodies that seem less optimal from our perspective and yet they thrive. [Octopus clip] Just because they haven’t developed any technological species yet, doesn’t mean they couldn’t in principle.

So what can we take away from all this? Even though bilateral symmetry possibly only originated once on our planet, its success here makes it likely that the same can be expected to evolve on other planets. This would then also likely lead to cephalization and therefore a head, though not necessarily a neck. Exactly how this pans out depends on the historical quirks of past evolution. In a future video I will talk about evolutionary contingencies and dive into more detail of the subsequent developmental constraints that shape the different animal bodies.

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