Most survivors of that progress trap became farmers — a largely unconscious revolution during which all the staple foods we eat today were developed from wild roots and seeds (yes, all: no new staples have been produced from scratch since prehistoric times). Farming brought dense human populations and centralized control, the defining ingredients of full-blown civilization for the last five thousand years.

The first trap was hunting, the main way of life for about two million years in Palaeolithic times. As Stone Age people perfected the art of hunting, they began to kill the game more quickly than it could breed. They lived high for a while, then starved.

Prof. Meiri: "Our study tracked changes at a much higher resolution over a considerably longer period of time compared to previous research. The results were illuminating: we found a continual, and very significant, decline in the size of animals hunted by humans over 1.5 million years. For example, a third of the bones left behind by Homo erectus at sites dated to about a million years ago, belonged to elephants that weighed up to 13 tons (more than twice the weight of the modern African elephant) and provided humans with 90% of their food. The mean weight of all animals hunted by humans at that time was 3 tons, and elephant bones were found at nearly all sites up to 500,000 years ago. "Starting about 400,000 years ago, the humans who lived in our region -- early ancestors of the Neandertals and Homo sapiens, appear to have hunted mainly deer, along with some larger animals weighing almost a ton, such as wild cattle and horses. Finally, in sites inhabited by modern humans, from about 50,000 to 10,000 years ago, approximately 70% of the bones belong to gazelles -- an animal that weighs no more than 20-30kg. Other remains found at these later sites came mostly from fallow deer (about 20%), as well as smaller animals such as hares and turtles."

Dr. Ben-Dor: "Our findings enable us to propose a fascinating hypothesis on the development of humankind: humans always preferred to hunt the largest animals available in their environment, until these became very rare or extinct, forcing the prehistoric hunters to seek the next in size. As a result, to obtain the same amount of food, every human species appearing in the Southern Levant was compelled to hunt smaller animals than its predecessor, and consequently had to develop more advanced and effective technologies. Thus, for example, while spears were sufficient for Homo erectus to kill elephants at close range, modern humans developed the bow and arrow to kill fast-running gazelles from a distance." Prof. Barkai concludes: "We believe that our model is relevant to human cultures everywhere. Moreover, for the first time, we argue that the driving force behind the constant improvement in human technology is the continual decline in the size of game. Ultimately, it may well be that 10,000 years ago in the Southern Levant, animals became too small or too rare to provide humans with sufficient food, and this could be related to the advent of agriculture. In addition, we confirmed the hypothesis that the extinction of large animals was caused by humans -- who time and time again destroyed their own livelihood through overhunting. We may therefore conclude that humans have always ravaged their environment but were usually clever enough to find solutions for the problems they had created -- from the bow and arrow to the agricultural revolution. The environment, however, always paid a devastating price."

A new study tracks the development of early humans' hunting practices over the last 1.5 million years -- as reflected in the animals they hunted and consumed. The researchers claim that at any given time early humans preferred to hunt the largest animals available in their surroundings, which provided the greatest quantities of food in return for a unit of effort.