While they may be outnumbered and outweighed by insects, the terrestrial world is really the kingdom of the vertebrates. Mammals, birds, reptiles and amphibians dominate most ecosystems. Yet, as Drew Harvell points out in her fascinating new book, the seeming diversity of the terrestrial vertebrates is deceptive. In fact all are contained within just one of the 34 groups of animals that live on our planet, and their many designs are really variations on a quite narrow set of themes. This means that all share bilateral symmetry – heads with eyes and brains atop bodies with four limbs that contain their organs.
By contrast, the other 33 groups of animals, the invertebrates, are astonishingly various in their design, with arsenals of adaptations and biological tricks. And while there are many invertebrates on land, it is in the ocean – the birthplace of all life on Earth – that their evolutionary innovations find their most extraordinary expression. Invertebrates subvert our assumptions about the basic nature of animals. Whereas animals on land are almost exclusively mobile, many marine animals, such as corals, sea pens, anemones and sea squirts, spend most of their lives anchored in one place, like plants; others do not breathe oxygen or even consume food in the form of organic matter. Invertebrates also possess abilities that challenge our conception of the boundaries between different kinds of organisms.
Harvell, a professor of ecology and evolutionary biology at Cornell University, writes vividly and with tangible delight about this strange world, zooming in on a series of the most striking of these animals. Along the way, she teases out not just the radical nature of their adaptations but also their potential to help open up new avenues for human innovation.
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