字幕表 動画を再生する 英語字幕をプリント (Stories from the Sea: How Life Came to Land) Life for my kind wasn't always this way. There was a time when no animals lived on land. All life was in the ocean. The ocean's where all of us animals got our start, more than half a billion years ago, This is where all animal body types -- or phyla, as scientists call them -- first evolved. You know, there are more than 30 animal phyla, but only a handful of major ones had what it took to do something completely daring: step out of the ocean, and on to dry land. So, which of these land-dwelling phyla first invaded the land? Was it me and my mollusc friends, with our amazing mantles and single foot? Perhaps the chordate crowd, with their notochords, segmented muscles and big bony skeletons. Or maybe those lowly annelid worms, with their powerful ringed bodies. Or did the Arthropods first make landfall, with their little flexible suits of armor -- their exoskeleton? Ah, yes. The arthropods. From crustaceans to millipedes, spiders to insects, the arthropods outnumber all animals on land. So what's their secret? Their exoskeleton is key, but here's the real kicker: jointed appendages. They're like little living Swiss army knives: antennae, multiple mouth parts, an obscene number of legs, if you ask me. There's one group of arthropods -- the insects -- that really rule the land. Three body parts, six legs, and an annoying tendency to take over. Sure, they have to molt to grow, but that doesn't seem to pose a problem. The insects even invented the first wings, and conquered the skies. Ah, well. So what if arthropods were the first to reach land and invent flight, a hundred million years before the rest of us? So what if they pollinate crops around the world, and make up 75% of all land animal species? And that for every single human, there's 200 million of them? Sure, those leggy arthropods may still be in the lead when it comes to conquering land, but we still rule in the sea. There are more species of molluscs in the ocean than any other animal phylum. We're just getting started up here on land. Besides, the seas are rising. Just give us some time. Who knows who'll end up ruling this ocean planet?