What evolutionary pressures would lead to Orcs?

Question

In my story Orcs, humans, and a bunch of other fantasy races live along side each other and i'm curious as to what evolutionary pressures would to a group of divergent hominids to evolve into Orcs?

The basics of my Orcs

• Can range in height from 161.5cm (5ft 3in) to 213.3cm (7ft) tall

• Are as intelligent as humans

• Both men and woman have tusks though men do have larger ones

• Are much stronger than human being slightly weaker than a chimpanzee but spend more time resting (not necessarily sleeping)

• Have a mildly improved sense of smell

• can interbreed with humans but the resulting offspring are always infertile (similar to mules)

• Have a symbiotic relationship with a type of algae not only allowing the orcs to become much stronger but also giving them their iconic green skin (no longer true)

Note: Magic does not exist in my story

Edit: after some contemplating i decided to remove the algae part for believably and now there gray hope that doesn't change things to much

Answer 1

Jungle Giving Way to Savanna

Your orcs share many similarities to humans in that they are both bipedal, roughly the same size and have evolved very high intelligence (for the animal kingdom). So therefore the pressures that made humans will therefore be similar to the ones that made orcs, as this is how convergent evolution works.

The Orcs ancestors were like those of humans; tree dwelling apes that needed to adapt to a changing climate that opened up grasslands. The proto-orcs got their humanoid build and intelligence from these analogous environmental conditions.

But there are notable differences.

Sexual Selection

The tusks of orcs evolved from lower canines that didn’t really serve a particular purpose, but were not harmful. Over time it was selected for sexually much like the antlers of a caribou, which is why they are more pronounced in males but not absent in females.

A Lusher Grassland

The savanna that the orcs evolved in was similar to man’s cradle in the Great Rift Valley, but with some notable differences. The rains were more frequent, leading to taller and more verdant grasses that limited endurance hunting like humans perfected. Therefore orcs evolved a greater sense of smell, in order to detect creatures hiding in the grass as it moved about, and to smell blood, as it couldn’t rely purely on good vision like early human hunters.

An unfortunate reality of the verdant plains for our proto-orcs was the presence of dangerous predators. Orcs evolved their greater strength, better smell and penchant for violence due to the presence of large predators that they needed to fight off at any given moment.

The Algae

This is the only part that’s tricky. Although there are mammals with algae in their fur such as sloths, no mammals have it in their skin and they definitely don’t provide energy. To have an animal take advantage of photosynthesis in its body is an incredible and one in a billion adaptation that is highly advantageous. Orcs got lucky and now get a little more energy from the sun via the microorganism’s photosynthesis producing sugars that the orc can absorb.

Answer 2

I think you need to thoroughly create and vet your entire humanoid collection and determine which might be related to which and those that aren't specifically related. In doing this you will define genetic connectors and disconnectors between each humanoid.

In looking at Earth history, you find homosapiens at some juncture mated with neanderthals, so DNA comes down to present day from that join. We do not know origins for those two lines. In your case, you are at that point where both existed and are defining what their relationships are (if any).

Why do your orcs have a symbiotic relationship with algae? Does this mean they also are water-centric where algae typically comes from? What is that relationship like? What benefits do both creatures get out of that?

Other than that single attribute, you have only defined a slightly different human with tusks. I think your other hominids are also important in defining these differences and if you map them out you will find the pressure points you need for your story. Dig into that algae vein and you will find the reason for your orcs.

Answer 3

No reason orcs couldn't still be green if you want - the algae could even be semi-parasitic, since orcs aren't known to be clean. More civilized ones could be grey. I agree, tusks make sense as sexual displays. The chimpanzee comparison is apt, as the difference between their musculature and humans is that they trigger more musculature at once (see hysterical strength) and human muscle is based on maintaining a lot of endurance. Your orcs would be lazy because they had to be. their muscles would burn out trying to do manual labor. They would be good conquerors but poor slaves (think Mongols). They would probably prefer cavalry or hobilar warfare to compensate for limited endurance. More overwhelming surprise attacks, fewer day-long battles.