Once we move away from microscopic organisms and move into the realm of complex-celled creatures of the deep, not to mention those who made it to the beachhead and onto dry land, it seems to me that almost all species have their reproductive organs placed at the back/lower portion of their anatomies.
Once propulsion came into the picture in the briny deep, I get why various sensory organs may have developed in the forward-facing areas. Early optic bundles, mouths, eventually gills, etc. All served the survival act of moving forwards.
Why the reproductive organs? I know, tons of species deposit eggs into silt and said eggs are then fertilized, etc.
But- next to finding and eating food to survive ( eyes, mouth, gills, etc ), making baby versions of yourself is kinda a driving force. Why, then, would so many species have developed to have their reproductive organs at the back/bottom of their body shape?
I’m trying to figure out an evolutionary advantage and am coming up empty-handed.
I think it’s a combination of a few things. First, it’s easiest to have the reproductive parts next to the waste-elimination parts. Second, if you have a complete digestive tract, it’s easiest to have the digestive-waste-parts on the opposite end of the body from the eating parts. Third, it’s easiest to have the eating parts on the front end of the body (meanwhile, it’s also easiest to have the sensing parts on the front end, and the thinking parts close to the sensing parts).
Beyond the points raised, it seems to me that having the baby making stuff located away from the fighting business end of the animal is good sense. Biting, swiping at things, ramming horns or antlers, etc. Even animals that make use of rear kicks do so in addition to front kicks and biting either in defense or while fighting for mates or otherwise tussling with their own species for territory or group status.
I wouldn’t say that’s WHY it’s in the rear since evolution placing it there happened long ago and before we probably saw some behaviors we do now but it seems like a pretty good reason why no animals saw it beneficial to move everything more upfront.
Because their distant common ancestor did, and evolution tends to conserve old traits. Moving them by now would have to avoid screwing up the many millions of years of later evolutionary adaptions layered on top of the developmental processes that results in them being there.
We all agree that having the sensory inputs at the front or top in the direction of movement makes sense. What does that imply? Waste elimination should go at the rear to avoid fouling or interfering with the senses. Locomotion goes on the bottom because that’s where the earth is. Nothing goes on the vulnerable top except defenses or camouflage. Most land animals are built on this template.
Where does that leave reproduction? I left it for last deliberately. Everybody “knows” that animals really only exist to produce more animals. But the reality is that the other systems I mentioned are used every single day. Reproductive systems are used only occasionally and, in most higher animals, require large amounts of space only in the female. Why should they take precedence?
The outlet, however, probably needs to be on the back or bottom because the baby has to drop to the ground. Since the inlet is also the outlet that’s where the whole shebang goes. QED,
Probably a major reason, too. Mother Nature tends to use the same playbook over and over, with a bit of stretching, duplication, and the occasional fade-out. We have two eyes like almost every vertabrate. Our fingers and toes are built on the same pattern, as our limbs are. The skeletal pattern harkens back to the earliest fishes with minimal variations. Evolution finds something that worked and never changes it. Accidental gene duplication gives us extras, like 5 fingers and toes on a limb and that stays if it’s useful. We get cones on our retina for light sensitivity, then evolution by accident makes one version sensitive to a differnt wavelength of light, and that’s an advantage, etc. Build on these accidents by selection, but major changes to the overall plan are rarer - which is why we don’t have eyes in the back of our head. Eyes are expensive to maintain, so perhaps 4 eyes are more trouble than the advantage they would convey.
Perhaps originally in fish, eggs were dropped and left behind. As a result, it makes most sense - more evolutionary advantage - for the eggs to come out the back as the fish swims away. This carried over into the messy birth processes on land being away from the food intake. Male and female genitatlia are based on the same master pattern, just develop differently, so located roughly in the same area - as little genetic differentiation as necessary to work properly.
Everything in evolution is a trade-off between cost and benefit of making changes.
I expect it goes back even further than fish. Most likely to Bilateria, the first of which were essentially worms that had a definite front and back. Almost all animals are descended from them.
Discussions of this topic often seem to gloss over what STM an important issue that would favor keeping reproductive organs near either the digestive/waste or sensory ones. Namely, localize all the holes in body regions that are easy to protect.
Body holes are vulnerable. They tend to have sensitive membranes that are more easily traumatized than ordinary outer integument, and delicate nerves that can be damaged. Keep all those fragile bits more or less in the same place(s)!
In humans, we have the sensory holes and the holes at the front of the digestive/respiration apparatus conveniently packed into the head, which also contains the heavily-shielded brain and is most effectively defended by the arms (and the mobility of the neck and trunk). Then we have the holes for the back end of the digestive tract and the reproductive organs conveniently tucked away in the crotch at the center of the body, where the movements of locomotion, manual activity, etc., don’t expose or affect them much. The crotch is also a region that it’s easy to add additional protective covering to, without impeding bodily functioning.
Voila, an anatomical hole-consolidation system that gives us only two high-vulnerability body regions. (Male humans, of course, carry the hole-consolidation process even one step further by using the same hole for both reproductive transfer and liquid waste elimination: nice tweak, guys.)
[ETA: I slightly changed that last sentence after posting, Marvin’s response below accurately quotes the original form.]
If it’s okay now that the factual question has been thoroughly addressed, it was certainly avoided by early Mother Church. Rather than consider the commonality of animal life, they preferred to keep the anatomical down at the level of “milk, milk, lemonade, ‘round the corner fudge is made,” with the arrangement of orifices set for moral reasons: Inter faeces et urinam nascimur “between poop and pee we are born.” The same principle that had interment in open view catacombs, for moral instruction as the putrefaction of vanity.
I tried deliberately to keep humans out of my discussion, as humans are an upright biped, unlike every other complex animal with the exception of some birds. Aspects of the skeletal system shifted with that move but not the placement of exterior systems. That can cause problems. How many threads have there been asking why the male genitalia are so exposed and vulnerable?
Humans do not, but what about other animals? From watching nature documentaries my impression is that the reproductive organs don’t need much protection. Attacks by predators on prey are usually on the broad thick sides that are easiest to grab. Maybe birds think to pluck out eyes but that is not a dominate strategy overall. Battles between males of the same species for superiority are usually head to head and that is indeed where horns are located, along with claws that can reach out.
Nor is the natural environment much of a hazard. Animals keep the “holes” off the ground, certainly, but otherwise can mostly ignore them. What would be a problem is if the locomotion system interfered, which is another reason for putting it at the back. As md-2000 noted fish just swim away from their roe, clean and easy. Birds and reptiles drop eggs. Mammals squat to deliver young. It’s counter-intuitive but true that the worst danger to the “holes” would be the animal itself, not the outside world.
