Life's unfolding is a tapestry in which every new thread is contingent upon the nature, timing, and interweaving of virtually all previous threads.
This is an extension of the idea that the origin of new life forms is fundamentally contingent upon interactions among previous biotas. As Stephen J. Gould described it, if one could rewind the tape of life and let events play out again, the results would almost certainly differ dramatically. The point of distinction here is a deeper incorporation of the connections inherent in the web of life. Specifically, the origin of new species is inextricably linked both to evolutionary history and to intricate ecological relationships with other species. Thus, speciation might be aptly termed "interdependent origination." So, for example, it is often said that the extinction of dinosaurs 65 million years ago cleared the way for the radiation of mammals and, ultimately, the origin of humans. Yet the degree of life's interconnectedness far exceeds that implied in this statement. Dinosaurs persisted for 160 million years prior to this mass dying, co-evolving in intricate organic webs with plants, bacteria, fungi, and algae, as well as other animals, including mammals. Together these Mesozoic life forms influenced the origins and fates of one another and all species that followed. Had the major extinction of the dinosaurs occurred earlier or later, or had dinosaurs never evolved, subsequent biotas would have been wholly different, and we almost certainly wouldn't be here to contemplate nature. An equivalent claim could be made for any major group at any point in the history of life.