Read Brain Rules: 12 Principles for Surviving and Thriving at Work, Home, and School for Free Online

collected from our ancestors could fit into your garage, with enough room left over for your bicycle and lawn mower. DNA evidence has been helpful, and there is strong evidence that we came from Africa somewhere between 7 million and 10 million years ago. Virtually everything else is disputed by some cranky professional somewhere.
    Understanding our intellectual progress has been just as difficult. Most of it has been charted by using the best available evidence: tool-making. That’s not necessarily the most accurate way; even if it were, the record is not very impressive. For the first few million years, we mostly just grabbed rocks and smashed them into things. Scientists, perhaps trying to salvage some of our dignity, called these stones hand axes. A million years later, our progress still was not very impressive. We still grabbed “hand axes,” but we began to smash them into other rocks, making them more pointed. Now we had sharper rocks.
    It wasn’t much, but it was enough to begin untethering ourselves from our East African womb, and indeed any other ecological niche. Things got more impressive, from creating fire to cooking food. Eventually, we migrated out of Africa in successive waves, our first direct Homo sapien ancestors making the journey as little as 100,000 years ago. Then, 40,000 years ago, something almost unbelievable happened. They appeared suddenly to have taken up painting and sculpture, creating fine art and jewelry. No one knows why the changes were so abrupt, but they were profound. Thirty-seven thousand years later, we were making pyramids. Five thousand years after that, rocket fuel.
    What happened to start us on our journey? Could the growth spurt be explained by the onset of dual-representation ability? The answer is fraught with controversy, but the simplest explanation is by far the clearest. It seems our great achievements mostly had to do with a nasty change in the weather.
    new rules for survival
    Most of human prehistory occurred in climates like the jungles of South America: steamy, humid, and in dire need of air conditioning. It was comfortably predictable. Then the climate changed. Scientists estimate that there have been no fewer than 17 Ice Ages in the past 40 million years. Only in a few places, such as the Amazonian and African rainforests, does anything like our original, sultry, millions-of-years-old climate survive. Ice cores taken from Greenland show that the climate staggers from being unbearably hot to being sadistically cold. As little as 100,000 years ago, you could be born in a nearly arctic environment but then, mere decades later, be taking off your loincloth to catch the golden rays of the grassland sun.
    Such instability was bound to have a powerful effect on any creature forced to endure it. Most could not. The rules for survival were changing, and a new class of creatures would start to fill the vacuum created as more and more of their roommates died out. That was the crisis our ancestors faced as the tropics of Northern and Eastern Africa turned to dry, dusty plains—not immediately, but inexorably— beginning maybe 10 million years ago. Some researchers blame it on the Himalayas, which had reached such heights as to disturb global atmospheric currents. Others blame the sudden appearance of the Isthmus of Panama, which changed the mixing of the Pacific and Atlantic ocean currents and disturbed global weather patterns, as El Niños do today.
    Whatever the reason, the changes were powerful enough to disrupt the weather all over the world, including in our African birthplace. But not too powerful, or too subtle—a phenomenon called the Goldilocks Effect. If the changes had been too sudden, the climatic violence would have killed our ancestors outright, and I wouldn’t be writing this book for you today. If the changes had been too slow, there may have been no reason to develop our talent for symbolism and, once again, no book. Instead, like Goldilocks and the third