When arachnologists set out to better understand how the world’s spiders were all related, they weren't quite prepared for what they found. Many researchers had thought that orb weaving—which yields those beautiful circular webs that hang between tree branches—was the pinnacle of spider evolution, making possible the rise of many of the tens of thousands of spiders that exist today. But a new family tree suggests it was hunting on the ground that led to a spider explosion, and that that orb weaving played only a minor role. A large team looked at 3400 genes from 70 species, using a relatively new technique for pulling out all the active genes from a genome to analyze. The new family tree shows that terrestrial spiders were the ones that really expanded in number about 100 million years ago, the researchers report online today in PeerJ. Most of these species, which include fishing, jumping, flower, and wolf spiders (above), derive from a single branch where males all have a tiny hook on each of the short front legs used in mating. The invention of feathers in birds, and the invention of flowers in plants made possible the rapid evolution of lots of new species. But it's not the mating hooks that make the difference for spiders, the researchers say. Instead, they think that the spiders stem from a single ancestor that gave up orb weaving in favor of ground hunting, where insect prey were more plentiful than in the air. Although there are 45,000 known species of spiders, the 70 represent a good cross-section, solidifying the spider family tree.