Humans co-operated with each other in hunting and fighting, but this co-operation occurred within groups of close relatives.

ECONOMIST: Economics focus

Campaigners fighting plans to close accident and emergency departments in north-west London have handed a petition in to Downing Street.

BBC: North-west London A&E fight goes to Downing Street

Once you bring forces close together in face-to-face trench line, house-to-house fighting, it negates the use of much air support, especially as both sides have a lot of the same gear and it's very difficult to tell between friend and foe.

CNN: Grange