My question is this: On account of the shapes living organisms have today, is any other way possible for life to exist (for example turtles look differently, humans have different shapes)? In other words, is there another way the organisms of today could look like?
Science fiction writers routinely construct biologies on other planets that are nothing like what we're familiar with. Based on silicon instead of carbon, trisexual instead of bisexual, deriving energy directly from electromagnetic waves instead of eating and breathing, even life that can thrive at the temperature of liquid nitrogen. or of liquid metal. Read James P. Hogan's novel
Code of the Lifemaker for a thought-provoking description of a planet on which all the life appears to us to be mechanical, yet they have invented "machinery" that looks organic. Each thinks the other is the remnants of the technology of a long-dead civilization that looked like them.
But we don't really know enough about life to postulate how many different ways it could form. All of these science fiction scenarios might be possible, or there might be some fatal flaw in the biochemistry that would prevent them from ever happening in real life.
The carbon-based life built upon DNA that we have here on Earth is the only type that we know for sure is possible. Anything else is speculation.
As for why the specific kinds of animals, plants, fungus, algae, bacteria and protists (the six Kingdoms of lifeforms in biological classification) look the way they do, there's no answer to that except that's the way it happened. Of course evolution is all about survival of the fittest, but it's also very much about randomness. Some mutations just happen to occur and some just don't, so we never get to see the results of the second category. There are many causes of mutations and it's just luck.
I'd like to know why Homo sapiens evolved such massively large brains. I think it has to do with sexual selection. Like a peacock's tail.
All of the primates have rather high intelligence, compared to many other mammals. While speed and strength are advantages for predatory species, most primates are not predators and other characteristics provide advantages for their lifestyles. The apes in particular developed high intelligence with which they form highly cooperative social groups that can find food efficiently, raise their young communally, and use teamwork to defend against predators.
The gorillas and chimpanzees developed even greater intelligence than the other apes, and this made them very successful in Africa. Eventually early species of humans branched off from the early chimpanzees, and their even greater intelligence was an advantage in finding food and outwitting predators. Successive species of humans had ever-larger brains, which they used to develop technology, such as building tools out of sticks and stones and taming fire.
Our ancestral species were primarily herbivores, like the gorillas and chimpanzees are today. However, the flint blades we invented allowed us to scavenge the bits of meat left over on the bones from other animals' kills, adding more protein to our diet. More protein allowed us to grow even larger brains, and more intelligence allowed us to build more clever tools and develop more complex social organization. Eventually we became the predators, killing our own meat instead of scavenging it. The final stage in our evolution was
Homo sapiens, the species of which we are all members. Our species lost the long intestines with the symbiotic bacterial culture that allows the other apes to eat leaves and digest cellulose, so with our incredibly well-made tools we became full-time carnivores, hunting to live. But unlike other predators, our success is not due to speed and strength, but rather to intelligence. Man is still the
apex predator in every ecosystem on this planet: we eat both bears and sharks.