Alien Biosphere Evolution #2: Building Body Plans

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! [IF YOU STEAL MY CONTENT I WILL FILE A DCMA!] 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. 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 providing 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 bodydivisions. 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. 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 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 1 opening, which is on their belly side. With their head they seek out food to move
their body over and ingest it 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 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. 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. 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 some more and dive into more detail… … of the subsequent developmental constraints that shape the different animal bodies. Until next time, cheers and bye, bye !


  1. It's worth noting that cephalization is compatible with radial symmetry. For instance, sea cucumbers have radial symmetry and still have a "forward" in which to crawl.

  2. Can you investigate whether higher intelligence is inevitable ?

    In a water planet, where the human body plan would not come into existence, would an intelligence water based creature grow intelligent ?

  3. Hi everyone! I have recorded audio for part 3 of this series and hope to get it out within a couple of weeks, so stay tuned!

  4. The interesting question is not "does it have to be like this?" but "How can it be?". The answer to the former is always no, but there are infinite answers to the later question. Using cratures from earth as a baseline when immagening alien animals is the safe option. We know it works and we imediately know what to expect of that creature. But being creative with the few rules nature gives us can result in much more interesting specimen than scaly spacedogs and cats with antennae. There is no wrong alien creature, just less likely ones.
    By the way: At least sea pigs (sea cucumbers with legs) evolved bilateral symmetry independently.

  5. There are non-axial symmetries. Consider an icosahedron like many viruses? It is certainly symmetrical but is not just symmetric on a plsne or axis but on EVERY plane or axis which contains a certain central point.

  6. More legs doesn't always mean faster movement. Ostriches are quite fast but have two locomotory legs, whilst millipedes can have hundreds but move slowly even relative to similar sized animals.

    So it is not so much the case that "legs are good, more is better" as that "more legs, simpler balancing, lower profile" and that each leg being large and long allows fast movement.

    This gravitates towards minimizing locomotory limb count. Maybe not down to 2 or 4, but not 20 for a fast land animal, as this would limit the ability to dedicate a lot of resources to each leg, which is required to make those legs large and strong.

  7. I can see a creature like a sea urchin being an intelligent being, or the shape anyway. One eye on a stalk, a round ball that sees in all directions with arms among the forest of spines, sea urchins already have this. Possibly the eye on a stalk would be a large compound eye with small simple eyes on arms like the pinchers on the other arms. The body plan of a urchin seems rather well suited for intelligence of course having no real brain and not being able to breath air are a handicap and well as using sea water for blood.

  8. 4:20 I wouldn't actually call that arrangement completely different. Just compare that head with the cephalothorax of the spider next to it, and it is still a bodypart with a bunch of nervous/sensory organs, a mouth and multiple limbs attached.

  9. As long as they can feed, think, reproduce and interact with their environment in a meaningfull way (prehensile tails, trunks, hands etc.) there is nothing restricting to a bodyplan for technologically advanced species.

  10. Just curious: have you ever done a video on potential trilateral symmetry?

    We have only one major example on earth (Tribrachardium)… but I'm curious if Trilateral Symmetrt would be worth it, at all? To me, it seems like a potential "step up" from having multiple eyes, like a spider… but it's something that fascinates me.

  11. You use Opabinia as an example of the alternate placement of the mouth. However, I would like to point out that the 'trunk' of Opabinia is just that, a trunk. The limb acts just like a pachyderm's (Elephant, Mammoth, Snufflupagus) trunk, grabbing food and transporting it to a mouth on the underside of the head.

  12. All of this stuff is so scary.. Life is so scary.. We're weird. We should wipe us all out, maybe Life it's a virus.. a cancer.

  13. Well, if we made Mars a giant Savannah and populated it with chimps and tigers, the chimps would evolve into more upright postures to look over the grass. Kinda like us.

  14. Yes, we have "front" and "end" because of forward movement but "top" and "bottom" we have because of gravity of our planet.

  15. Interestingly recent study of the genomes and life cycles of cnidarians have revealed that the major body axis and bilateral, or at least tightly cylindrical with well defined head) body plan have deep roots predating the division of bilaterians and cnidarians which diverged according to molecular clock estimates around 721 Mya. Thus it may be that radial symmetry was a secondary adaptation to a sessile mode of life as both cnidarians and echinoderms appear to have evolved this symmetry from a primarily bilaterally symmetric ancestor.
    Hard to say where the other symmetries seen in the Ediacaran biota fall with respect to these other than they probably occurred prior to these groups split. One example Dickinsonia may be related to the Placozoa and appears to have possessed nearly bilateral glide symmetry which may share a common origin since Placozoa show many signs of secondary losses. I do find it quite interesting that the major divergence between Cnidaria and Bilateria appears to have occurred well within the Sturtian glaciation. Given that distinction and the first large metazoan fossils around 9 million years after the aborted snowball Gaskiers glaciation. This relation between the planet nearly freezing over and multicellular life appearing raises the interesting possibility that it may serve as part of the answer to the Fermi paradox as there is evidence for an earlier radiation of apparently multicellular fossils around 2.1 Gyr directly following the first snowball earth event which itself directly followed the Great Oxygenation Event. This first radiation failed to survive the subsequent dearth of oxygen which persisted over the following 1.5 billion years until the Neprotozoic ice ages further strengthening this link.

    Recent work has suggested that the evolution of metazoan like organisms may have been driven to adapting to live within the cracks fissures and meltwater seeps within the cryogenian ice sheets which would have been oxygen and nutrient rich near the equator due to the constant sublimation and melting of ice. This is still speculative based off current evidence and might be impossible to ever confirm since ice dwellers would be unlikely to fossilize but it raises interesting prospects for life in the galaxy!

  16. In a nutshell, evolution works with what it has and what's successful!!



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