I understand that humans have been adopting for a while now, but when is the earliest sign of it? And also how did it differ from todays standards of adoption?
Thanks
Do you mean legal adoptions, or simply a child being cared for by someone who is not their biological parent?
Adoption is older than recorded history. It was well known and widely practiced in the ancient near east. Just for example, numerous instances of it are mentioned in the Hebrew Bible, including one instance of a disabled person (named Mephibosheth) being adopted and cared for as an adult, out of respect for his deceased father. That was circa 1000 BC, but there are earlier examples, e.g., Jacob's wife apparently pre-arranged to adopt her servant's child as her own (a surrogate situation), which occurred perhaps as much as a thousand years earlier (although the written account dates to more like 1500 BC). Moses was also adopted. I expect someone else can come up with an even earlier example.
Oedipus was also said to be adopted, but I don't know which version of the tale first included that detail, so it's hard to put a date on it. I suspect it may be more recent than the Hebrew examples, but I don't have a reliable source for that.
A noteworthy thing that all of these examples have in common is, the texts do not waste any time explaining in detail what it means for a person to be adopted. Apparently the audience was expected to already be familiar with the concept.
Not exactly what you were asking, but to address the last part of your question, adoptions in ancient Mesopotamia often came as a sort of insurance policy -- the adoptee had to provide for their new parents a set amount or provide a slave as a substitute. It was often to make sure that a widow would not be left indigent.