I don't know who the "it" refers to in your post above.
The reason that "but" is the right answer is the comma after "told."
If you want to use "however", then change that comma to a full stop or a semicolon.
Note also that the first comma in the text is incorrect. It makes the text a comma splice.
Here are your alternatives:
The Piraha tribe may be small, as it only has about 200 ... etc.
The Piraha tribe may be small. It has onlyabout 200 ... etc.
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