I would actually suggest that contributors should mail a drop of blood or tear as a proof they are humans:

https://lwn.net/Articles/1074534/

Arias: Human proof for FOSS contributions

Rodrigo Arias Mallo, maintainer of the Dillo web browser, has written a blog post with a propos [...]

LWN.net

@EvenRouault If I may chime in from across the web: combined with the concept of these rent-a-human websites that went up in the beginning of the claw bot, this makes for a great dystopian sci-fi setting!

At least from the perspective of 10 years ago.