What type of behavior was among the first to evolve among the earliest (most primitive) animals?

Solution:

Primitive animals are ones that have not changed dramatically over the millennia and remain very similar to their ancestors.

The first members of the human lineage lack many features that distinguish us from other primates. Although it has been a difficult quest, we are closer than ever to knowing the mother of us all. Until recently, the evolutionary events that surrounded the origin of the hominin lineage - which includes modern humans and our fossil relatives - were virtually unknown, and our phylogenetic relationship with living African apes was highly debated. Gorillas and chimpanzees were commonly regarded to be more closely related to each other due to their high degree of morphological and behavioral similarities, such as their shared mode of locomotion - knuckle-walking. But with the advent of molecular studies it has become clear that chimpanzees share a more recent common ancestor with humans, and are thus more closely related to us than they are to gorillas (e. g., Bailey 1993, Wildman et al. 2003). The similarities between the living African apes were thought to have been inherited from a common ancestor (=primitive features), implying that the earliest hominins and our last common ancestor shared with chimpanzees had features that were similar, morphologically and behaviorally, to the living African apes (Lovejoy 2009). With the discoveries of the earliest hominin species discussed below, it is now possible to critically examine these assumptions.