The most important benefit might have been the high content of iodine, which is usually insufficient in terrestrial food. Obviously they did not have iodized salt, like us, to provide the required iodine.
They might have noticed that eating a little amount of seaweed will prevent illness.
For omega-3 fatty acids, they had much better food sources in bone marrow or in fish (the multicellular algae have very low content of fatty acids; those that are used for omega-3 fatty acid production are unicellular and they are not algae, even if they are named "algae" in advertisements and on labels, because it is believed that something like "straminipiles" would be incomprehensible for laymen; even many scientists misspell "straminipiles" as "stramenopiles", only "Straminipila" is correct Latin).
The vitamins were also abundant in their food, only iodine is deficient in all terrestrial food.
> Obviously they did not have iodized salt, like us, to provide the required iodine.
Some places still (mostly) don’t. A notable exception (I’m sure there are others but not as well reported) is Israel where iodized salt isn’t a regulatory requirement and not very popular, with (predictable) widespread deficiencies:
Which makes me think that seaweed was staple food at some time right? We wouldn't have evolved to need more iodine than we can get in our environment otherwise...
According to WHO estimates [1], a quarter of Africans have goiters so I think it's safe to say that archaic humans suffered from chronic iodine deficiency.
Evolution works through reproduction so any condition that doesn't impact survival until sexual maturity is unlikely to face any evolutionary pressure. Since humans are tribal and took care of their offspring in group settings, survivability after procreation also has little effect.
> any condition that doesn't impact survival until sexual maturity
Diseases that impact even just appearance can surely make you less attractive as a mate though. And larger goiters can impede breathing or swallowing. So I'd be suspicious of the idea that we've been significantly deficient in iodine for extended periods of our evolutionary history.
Goiter is usually not immediately lethal so for most of human history, people afflicted with it just kind of lived with it which likely meant there wasn't that much selective pressure against it. Consuming thyroid glands of terrestrial animals (usually ruminants) was an effective treatment of iodine dificiency before the modern nutritional sciences discovered the relationships between goiter and iodine deficiency: https://en.wikipedia.org/wiki/Goitre#History
Ruminants are more efficient at absorbing nutrients from plant material due their digestive system, and this is how they get iodine in non-coastal areas.
All vertebrates have thyroid glands and there is a very similar organ in marine chordates: https://en.wikipedia.org/wiki/Endostyle. So this is an organ that likely emerged when our biological ancestors still lived in a iodine-rich environment.
I agree with you. I want to mention Sea salt has iodine very often and was a very common source for other kinds of salts besides sodium chloride. We started iodizing salt in response to electrical methods for extracting pure NaCl becoming popular.
They might have noticed that eating a little amount of seaweed will prevent illness.
For omega-3 fatty acids, they had much better food sources in bone marrow or in fish (the multicellular algae have very low content of fatty acids; those that are used for omega-3 fatty acid production are unicellular and they are not algae, even if they are named "algae" in advertisements and on labels, because it is believed that something like "straminipiles" would be incomprehensible for laymen; even many scientists misspell "straminipiles" as "stramenopiles", only "Straminipila" is correct Latin).
The vitamins were also abundant in their food, only iodine is deficient in all terrestrial food.