You do realize that every society before more modern times would not keep a physically or mentally handicapped child (few cases where some families would keep them). Most would kill them by leaving them on a mountain which was common practice in the old days. The difference is that as society changed so did our ethical codes as leaving physically or mentally handicapped children on a mountain is no longer acceptable, but even today it still happens were people abandon babies for different reasons.
Comparing us to animals isn’t fair comparison as animals don’t possess much (or any) higher thinking to ponder if it is ethical they just run off instincts mostly. If we went back to caveman days then we would do the same as them as it comes down to who is fit to survive.