How could a carbon-based mechanical lifeform evolve naturally?

Question

Imagine that there is an inhabitable planet that has just been discovered by human colonists. All of the unicellular life and some multicellular life such as plants is normal carbon based life made of cells. Various investigations confirmed this is the way things evolved.

However, the animals are thought to be robots designed by another species at first because they seem to be clearly mechanical. Where in this case mechanical should be taken to mean that movement and digestion are achieved via gears pulleys and other macroscopic discrete parts.

(I am aware that this is unlikely to happen but just think of a plausible natural evolutionary pathway by which this scenario could occur.)

What might be a plausible evolution pathway to having a life form with mechanical (gears etc) components?

Answer 1

M S has a great answer, specially since:

the reason we almost never see these structures in real-world animals, at least for wheels and pulleys (and freely rotating gears), is that they require the structure to be disconnected from the rest of the animal's body, which means the part cannot receive nutrients.

So my take on it is that what you perceive as a single specimen from a distance is actually a set of different creatures in symbiotic relationship. Some parts may even be whole colonies of single cellular organisms, or microscopic animals or plants.

In our own world there are some species of shrimp that swim into sponges as larvae, and then grow to be larger than the pores. They become trapped inside a sponge for life as adults. An alien might see the shrimps as free floating organs of the sponge at a first glance. Food for thought.

Another possible example: many birds swallow rocks. Those rocks are stored in a pouch close to the stomach, and are used to smash food - same role as teeth in mammals. Imagine that some sort of organism evolved to be swallowed by birds like that. Perhaps a plant whose seeds are really hard. When the bird eats, the seeds are immersed into some nutrient that is rare in the soil. When the seeds eclode they just become soft and come out of the south end of a bird flying north, then land on dirt where they can sprout. The bird then goes looking for new seeds to serve as internal teeth. If the seed develops gear indentations over time inside the bird, it may be even more useful for mastigation. This benefits both species, thus being favored by natural selection.

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After writing my ideas above I remembered a fictional species from His Dark Materials. Mulefas are quadrupede beings who harvest discoidal fruits from their homeworld to use as biological wheels. On top of being perfectly circular with a hole in the middle, the fruit also has an oil which lubrificates the surface which stays in contact with the mulefas' legs. The mulefas' legs in turn evolved to be able to grasp the fruit in a position that facilitates going around like a biological bycicle.

Answer 2

I won't be able to provide a particularly comprehensive answer here, but I'll throw in my two cents.

In one of my worlds I came up with the idea of an ecosystem filled with animals that, like you describe in your question, are 'mechanical', utilizing structures like gears, wheels, springs, and pulleys in their biology. As I understand it, the reason we almost never see these structures in real-world animals, at least for wheels and pulleys (and freely rotating gears), is that they require the structure to be disconnected from the rest of the animal's body, which means the part cannot receive nutrients. This adds a significant, and potentially insurmountable, layer of evolution complexity, as the part needs to be grown from something like keratin and then disconnected. Resultantly, legs dominate and likely prevent such unnecessarily complex structures from ever evolving.

The way this is overcome in my world is through the magic system, which allows an animal to use aura (magic stuff, basically mana) to transfer energy to body parts disconnected from the circulatory system, allowing separate body parts to grow and heal. So, if the world you are building is one in which magic exists, making 'the ability to transfer energy from one part of the body to another' an aspect of the system seems the simplest way, as it drastically reduces the barriers toward the evolution of mechanical structures.

If your world does not have magic, the only method I can think of would be for your animals to have structures that let them transmit and receive radio waves or microwaves, which would let them transfer energy to disconnected body parts. On the receiving end, I would expect something somewhat analogous to chlorophyll, in that there is a compound that converts incoming electromagnetic energy (photons) into chemical energy. I have no idea what the transmitting end would look like. You should also note that this suggestion is incredibly handwavey, as there would be massive conversion losses in the process that would make it unlikely to evolve (and no doubt issues with receiving low-frequency photons). But at the very least, it's an explanation that sounds somewhat plausible. It would also have interesting implications for how your animals could communicate, as they may do so using radio waves.