I would imagine that, after multicellular life capable of sexual reproduction developed, there was an incentive for creatures of all kinds to develop techniques to attract mates. And before eyes became a thing, I would imagine that things like scents were (and still are for many species!) important for attracting mates. It definitely goes far beyond mammals, the development of visual, olfactory, and other sensual features for attracting mates is important for all kinds of species that sexually reproduce.

But looks aren’t just for showing off to mates. They might also serve other purposes, like warding off predators or protecting itself from the environment. For instance, our eyelashes were developed to protect our delicate eyes from sand, dust, and other small particles, and the colourful appearance of some frogs indicate that they are poisonous.

Our looks are also how we identify others. For social creatures like us humans, prominent features like our eyes, hair, and ears are used to figure out who you are interacting with. This isn’t unique to humans and isn’t exclusive of visual features either. Dogs can distinguish between trusted individuals and strangers using a combination of sight and scent. Whales have strong sense of family and can identify others through, among other ways, their calls. Dolphins are similar with a pattern of chirps to identify their friends and family.

But if you really want to be pedantic, you could argue that the “look” can be defined as the physical (outer) shape of an organism, in which case the shapes of the receptors, cell membrane, etc. of unicellular microorganisms can be the different “looks” that have developed for different purposes. This would include viruses too, which are only partly alive, which might mean that this idea of “developing looks” could extend before life itself existed!

I didn’t do Biology in school so don’t quote me on any of this!