This post (inspired by Galina L) will be a trip down food-chain lane, starting at the bottom and working upwards. Humans are at the top (but not always).
1) Bacteria: The Vitamin B12 produced by certain bacteria is the only usable form that vegans will consume, so vegans should either not wash root veggies before cooking, or they should supplement with bacterial-sourced B12.
2) Algae: The EPA & DHA in algae are the only forms that vegans will consume, but beware. EPA has five C=C double-bonds and DHA has six, which makes them very unstable. EPA & DHA that are not inside a plant/animal should be kept cold and away from light.
3) Fungi & Yeasts: I don't have much to say about these other than they are a good source of B Vitamins other than B12. Mushrooms are tasty. Ditto yeast extract (though you either love it or you hate it). Quorn is made from a fungus.
4) Plants: The hotter the climate, the higher the saturated fat content of a fruit/nut/seed. Coconuts have the highest saturated fat content (~91%). The hotter the climate, the lower the pufa content of a fruit/nut/seed. Coconuts have the lowest pufa content (~2%). See the Comparison of dietary fats chart below.
Olives grow in many countries, some hotter than others. The Solesta EVOO that I buy from Aldi has a pufa content of 6.6%. However, some olive oils can have a pufa content as high as 20%.
5) Krill: The EPA & DHA in Krill oil can be consumed as a dietary supplement. The warning that applies to algal EPA & DHA also applies to Krill oil. Krill oil is a good source of Astaxanthin.
6) Fish: Fish that live in warm waters are low in oil (the oil in their livers is rich in Vitamins A & D). Fish that live in cold waters need antifreeze. EPA & DHA have very low freezing points (-54°C & -44°C respectively). The EPA & DHA in Fish oil can be consumed as a dietary supplement. The warning that applies to algal EPA & DHA also applies to Fish oil. See When it comes to fish oil, more is not better. Large fish (e.g. sharks) can eat us.
7) Amphibians: Frog's legs are a delicacy in some countries. What amphibians do you like to eat?
8) Reptiles: Turtle soup, anyone? Large reptiles (e.g. crocodiles) can eat us.
9) Marine mammals: As these are warm-blooded, they have a lower pufa content than fish. However, blubber is cold on the outside, so it's high in EPA & DHA. Large marine mammals (e.g. Killer whales) can eat us.
10) Mammals: As these are warm-blooded, they have a lower pufa content than fish. The pufa content depends a lot on what the mammal eats. Ruminants can eat a wide variety of diets and still maintain a reasonable omega-6:omega-3 ratio in their body-fat. Large mammals (big cats, big dogs and big bears) can eat us.
11) Humans: Many years ago, we ate anything edible that was around, lived outdoors and moved ourselves around using our muscles. We've done well, without modern medicine to save us from infections & injuries (and maybe difficult childbirths).
Nowadays, we eat what's in the shops, live & work indoors and have labour-saving gadgets to do our physical work for us. We're having problems, despite modern medicine.