After our planet cooled from a glob of molten space dust, survived the collision（碰撞） that made（使） the moon, and endured（忍耐） 300 million years of bombardment（炮击） from space rocks, life showed up about as soon as it could, but for the next 2 billion years, it got stuck, just single cells（细胞） living the single life. Until around one and a half to two billion years ago, when life started sticking together. Some people would say single cells are still the dominant（显性的） life form on our planet. But if bacteria（细菌） and archaea have done so great living alone, why and how did some organisms make the jump to living together? We can count eight major transitions（转变） in evolution（演变）, from the first replicating（复制） molecules all the way to societies and the language. But of all of them, the invention of multicellular（[生物] 多细胞的） life might have been the easiest.

  Because it didn't happen just once or even twice, we think it happened more than 30 times. The multicellular mashup that led to you or your dog was different from the ones that led to seaweed, sequoias（ 美洲杉）, and mushrooms. In the game of evolution, organisms are always looking for unoccupied（空闲的） territory（领土）, new niches（壁龛） to fill, and being bigger than the biggest thing around is always an open opportunity. The bigger you are, the more things that you can eat and the fewer things that can eat you. But one cell can only get so big before the harsh（严厉的） realities of surface area and volume（卷） make（使） life more trouble than it's worth. If you can't get enough nutrients or move things around the cell, eventually cells have to band together.

  Being bigger by getting multicellular brings other advantages. Your insides are protected from the outside so you can survive in rough neighborhoods where you couldn't before. You can live longer because little bits of you can die and be replaced. And the cells that will become the next generation are tucked（打褶裥） away, safe in sound away from that cold harsh outside world.