How small could humans be and develop a similar civilization?

Looking it up I see that cell death is real and huge. Never knew that!

For anyone else interested in the details if that culling:

I am still skeptical of that quiet bit. As the above covers in detail a large part of the culling occurs specifically to younger cells that are quiet, more precisely that activity triggers factors that promote survival and those cell that fail to become active therefore fail to get those growth factors and die off. The culling generally occurs at around birth.

Yeah, on reflection, quiet is a bit of an overstatement. Conduction is slower, and in the absence of significant mylenation function is not as strong. It seems that there is a bit of a chicken and egg. There may be an intrinsic phase where mylenation starts, and an adaptive phase where more axon signalling begets stronger mylenation.

Hmmm…I am not entirely confidant that is a correct statement. Songbirds have quite a slew of potential predators. Including a few specialists (some birds in particular), but also many, many incidental predators from felids to foxes to mantids to mustelids.