The trends and changes that made us human did not develop in isolation. There are some interesting connections between the four major trends.
Legs, hands, stone tools and diet
Our ancestors started on the path to becoming human when they began walking on two legs. The hands were now freed for other tasks but millions of years passed before stone tools began to be manufactured. The earliest known archaeological sites contain many flaked stone tools alongside collections of animal bone fragments. Examination of these tools and bones indicate that the tools were used to obtain meat and marrow from the bones.
Diet, teeth and brain size
Brain size began to increase about two million years ago and this may be linked to improved diets containing more meat. Isotope studies on teeth suggest meat was becoming a significant part of some hominin diets by about 2.5 million years ago. The change in tooth and jaw structure about this time also suggests a dietary change with more reliance on meat, as does the elaboration of tool technology.
The brain requires substantial amounts of energy (about 20% of dietary energy intake) and could only increase in size if its increased energy needs were met. Meat is a concentrated source of calories, protein and fat, providing a rich source of energy in which to sustain or develop a larger brain.
One side benefit of a shift to more meat in the diet is more free time. A herbivore spends a much greater percentage of time feeding compared to a carnivore of the same weight. For instance, a 35 kilogram australopithicine that included significant amounts of meat in its diet would shift from about 6 hours feeding time per day to only to 2 hours, freeing up large amounts of time in which to develop social and learning skills.
Tools, diet and tooth size
Improved technology and a greater reliance on meat in the diet may have affected tooth size since foods now needed to be chewed less. This is reflected in the reduced size of the premolars and molars – the teeth responsible for grinding food.