What would alien life look like?

[Anomalocaris]

Excellent points. Because even if we found a ‘twin Earth’ (and we haven’t, regardless of what appears in popular journalism), there’s simply no way of knowing that evolution would follow the same paths as it has on Earth.

Given the staggering variety of life here (a rose looks nothing like a rhinoceros), speculating about extraterrestrial life is just that - speculation.

Boru

But Protistas have microscopic life that take on the function of plants, animals, and fungai, only they're not related to any of those things beyond them all being eukaryotes. I think we'd find plants, animals, and fungai on an alien world, based on that.

I think life on other planets with conditions and chemistry broadly similar to earth probably will evolve organisms broadly similar in form and physical construction to bacteria and archea. But the structural and metabolic chemistry may well be very different.

But beyond simple prokaryotic like cells, I think any entirely independent line of evolution, even if it takes place in environment and chemistry similar to earth, would result to more complex cells or organisms that are quite different in all ways, including metabolism, structure, manner of reproduction at cellular level, from those on earth.

I think evolution of eukaryotic cells was a difficult and highly contingent step. I think the current majority view is eukaryotic cells formed by process of endosymbiosis of two or three different types of bacteria or archea cells. If I am not mistaken, all eukaryotic cells appear to descend from a common eukaryotic ancesters, implying the successful phylogenies resulting from endosymbiosis event occurred relatively few times in the history of life on earth and may be a difficult thing to pull off. That implies the biochemistry of all eukaryotic organisms on earth, including plants, animals and fungi, all depended heavily on the contingent circumstances of a singular or a comparatively small handful of particular endosymbiotic events.

This implies to me that if we were to wind the clock back on earth to before the first eukaryotic cell, and let time run forward again in its normal way, similar eukaryotic cells capable of leading to our complex animal, plants and fungi may not have a high chance of arising again even on earth.

So I suspect life on other planets, even if the physical and chemical environment were very like those on earth, would be unlikely to follow similar divisions as life on earth even at the most basic level of fundamental cell structure and organization.