How the Land Became the Locus of Major Evolutionary Innovations

Abstract

Life originated in the sea and evolved its early metabolic pathways in water [1, 2]. Nevertheless, activities of organisms on land have influenced and enriched marine ecosystems with oxygen and nutrients for billions of years [3-7]. In contrast to the history of species diversity in the sea and on land [8-10] and the flows of resources within and between these two realms [11], little is known about the times and places of origin of major metabolic and ecological innovations during the Phanerozoic. Many innovations among multicellular organisms originated in the sea during or before the Cambrian, including predation and most of its variations, biomineralization, colonial or clonal growth, bioerosion, deposit feeding, bioturbation by animals, communication at a distance by vision and olfaction, photosymbiosis, chemosymbiosis, suspension feeding, osmotrophy, internal fertilization, jet propulsion, undulatory locomotion, and appendages for movement. Activity is less constrained in air than in the denser, more viscous medium of water [9, 12-14]. I therefore predict that high-performance metabolic and ecological innovations should predominantly originate on land after the Ordovician once organisms had conquered the challenges of life away from water and later appeared in the sea, either in marine-colonizing clades or by arising separately in clades that never left the sea. In support of this hypothesis, I show that 11 of 13 major post-Ordovician innovations appeared first oronly on land. This terrestrial locus of innovation cannot be explained by the Cretaceous to recent expansion of diversity on land. It reveals one of several irreversible shifts in the history of life.