"Our first evidence of human life is a few stone tools." Is this sentence correct? - Yes. Could ARE be used instead of IS? - No "evidence" is singular. . Our first answer is correct, even if it just copies the responses that went before from people who for some unknown and unfathomable reason chose not to "answer" but to "comment".

The subject of the statement is 'evidence' and is singular, hence the use of 'IS'.

Our first evidence, the word "evidence" is singular, that's why you say "is" , as you are regarding to the word "evidence". But if you started the sentence vise versa , Few stone tools ARE our first evidence of human life, then "are" can be used.

You would use 'is' because it's talking about evidence, and evidence is singular and is an uncountable noun.