I think we have more than a sample of one instance to look at though. The distinctive thing about humans is more that we operated collectively according to social rules, than pure intelligence.*

Due to evolution, the coordination of individual entities has happened repeatedly. Individual molecules became living cells, some suggest living cells merged into cells with mitochondria, viruses and cells merged, single cells merged into multicellular organisms, insects organized into social colonies, and now primates, i.e. humans have also organized into social colonies.

This doesn't prove evolution always creates social multicellular life, but it means (IMO) there is a clear pattern that provides a reason to assume it does short of specific reasoning or evidence about a barrier - why should the bacterial level be unique, and which bacteria would be the highest level?.

*I don't think you can separate intelligence from a social context. What we think of intelligence is closely related to things like the willingness to give up current rewards for long term ones. But it's not "smart" to do that unless you have justified faith that you live in a stable society where people are trusted to follow social rules.

There are a couple of most plausibly hard steps: the origin of life up to just before bacteria split from archaea, and the origin of eukaryotes, which happened just once after something like 2 billion years. But I had in mind something cheaper more like scaling up the idea of http://www.karlsims.com/evolved-virtual-creatures.html. Intelligence has evolved in animals as unrelated as octopi and us.

In an artificial setting you can manipulate the conditions to favour whatever you want.