Maybe it's a dumb question, but how do we know these biomarkers weren't from eating fish that are the seaweed? If the biomarkers are able to remain detectable for thousands of years without breaking down, perhaps they're also able to accumulate in fish? I don't know anything about it so my idea is probably wrong, but I'm surprised the article doesn't mention it since it seems like an obvious thing to wonder.
I think they kind of cover that: > They found the chemical biomarkers of seaweed and aquatic plants in 26 samples, which suggests that early humans were eating—or, at the very least, chewing—these bounties from the sea Like, yeah maybe the fish they were eating was covered with seaweed, but unless they were popping the stomach/intestines into their mouth... for the parasite load(?), the seaweed in the fish is digeste…
And the article doesn't make any mention of which specific biomarkers they chose or whether they're affected by digestion. They're obviously pretty hardy compounds to begin with if they're not broken down by saliva to begin with, or thousands of years of natural decay.
And it does seem kind of akin to carbon-14 dating. It's not like they found actual recognizable pieces of the plants in between their teeth or anything.