Who was infected first?

Blair was infected first. I know, it's pure speculation but it's still fun to speculate!

Dunno- but one thing that always seems to defy logic is how the thing can take over anyone without anyone knowing it's happening at the time. The takeovers that we do see on screen have always been extremely noisy and MESSY. I'm guessing it took a minimum of 30 min for complete assimilation, and in that close, cramped station there is no way it could go on unnoticed.

Btw- If both Blair and Fuchs said that the thing needs to be alone and in close proximity to the victim in order to absorb them...why does the dog-thing try to absorb 4-5 dogs at once?

I think Palmer was infected first (the shadow on the wall), and Norris second via contaminated food. I think Norris was still human up to the point where he had his 'heart attack'. I think Norris's chest pains were really the Thing eating away his innards, then when it had assimilated enough, Norris collapsed and died. Between the time he collapsed and the defibrillator, the Thing was hurriedly assimilating more and more of Norris's body, until it had enough to attack.

Blair first, and very early.

The assimilations happened while Macready and Dr. Copper went to the other camp( an hour there, an hour back, enough time for at least two people to be taken over). I know Blair was taken over early, but I believe Norris was as well. I think Palmer was taken over after Blair " went crazy". Blair was sneaking back into the camp and stealing parts for his ship ( not as evident in the film, but elaborated more in the novelization). The men didn't notice because they were pretty much grouped together after a certain point. One thing you can't deny is that the thing was going after the scientists in the group first ( Blair, Norris, Bennings, Fuchs) and only imitated Palmer because he was a helicopter pilot.

If the thing can infect hosts on a cellular level anyone touching it could have been infected.
My theory is that both Blair and Fuchs were infected that way (probably during the autopsy). Fuchs burns himself, Blair loses the plot.
I'm also inclined to think the infections and assimilations are all random. Apart from Blair's escape ship there's little evidence of a plan or strategy from the creature.

Palmer was taken over first. You see this when the dog walks into his room in the beginning. Then Norris. Then Blair, not Blair first.
There are clues throughout the movie that can tell you when a character was transformed. Watch it again. The imitation versions always give themselves away by their suspicious behavior. Palmer says he ain't going with Windows, because everyone was suspecting Windows was an imitation. Palmer-thing blends in with others by agreeing with the suspicion and casting doubt on Windows.

What about Norris? Remember when Mac wanted someone to hold the keys. Norris was already the thing here. Norris got nervous and said, "Why me?". By shifting focus, blending in, that's how the thing always survived and slowly took over its areas.

Then Blair. I think Blair was taken over slowly starting with the autopsy. Watch it again. He clearly states that the organism appears to be dead, but the cells are still active. WATCH - He touches the thing with the pencil eraser, and then he touched his lips as he was thinking about it. It started transforming him then. So, remember what I said about the imitations casting doubt on others? Blair-thing comes out swinging, literally. He killed all the dogs, like a non-alien would. He destroyed the chopper and radio (he was going to use it for making his ship and it prevented people from leaving.) But this made him look like a crazy human, AND it forced him into isolation. LIKE the thing wanted. Remember, it could be part of a whole, and each piece will do what it has to to break off and survive, right?

That's what the isolation and crazy behavior was for: to break Blair-thing off, so it could work on its escape. While in the shed, he was digging a hole and putting together its ship.


 <---Bella's "Kiss Me" Face

Norris and then Palmer, Palmer's longjohns were shredded by The Thing.

If we assume that the first assimilation that we see is the shadow on the wall that the Dog-Thing walks in on, it's Palmer.

The filmmakers said that the scene was originally filmed with David Clennon (Palmer) in the room, but then he was replaced with a stunt double because his silhouette was too obvious. If we knew it was Palmer from the shadow profile, the shock of reveal on the couch wouldn't be there.