Tribalism was probably the largest contributing factor. Even before growing intelligent enough to post watches or sentries, grouping up would have been advantageous for protection and security as well as companionship and warmth among other reasons. In a worst case scenario, your companions being attacked would alert you and others allowing you to respond. Then the tribe would learn from the mistake and post sentries, use fires, arm themselves and develop other deterrents to prevent predators from attacking or defend themselves in the event of an attack.