5Peter Medawar's 'An Unsolved Problem of Biology' 1 was one of several formal attempts to 6 provide an explanation for the evolution of senescence, the increasing risk of mortality and 7 decline in reproduction with age after achieving maturity. Despite ca. seven decades of 8 theoretical elaboration aiming to explain the problem since Medawar first outlined it, we 9 argue that this fundamental problem of biology remains unsolved. Here, we utilise 1 0 demographic information 2,3 for 308 multicellular species to derive age-based trajectories of 1 1 mortality and reproduction that provide evidence against the predictions of the classical, still 1 2 prevailing, theories of ageing 1,4,5,6 . These theories predict the inescapability of senescence 1,4 , 1 3 or its universality at least among species with a clear germ-soma barrier 5,6 . The patterns of 1 4 senescence in animals and plants that we report contradict both of these predictions. With the 1 5 largest ageing comparative dataset of these characteristics to date, we build on recent 1 6 evidence 7,8 to show that senescence is not the rule, and highlight the discrepancy between 1 7 existing evidence and theory 7,8,9 . We also show that species' age patterns of mortality and 1 8 reproduction often follow divergent patterns, suggesting that organisms may display 1 9 senescence for one component but not the other. We propose that ageing research will benefit 2 0 from widening its classical theories beyond merely individual chronological age; key life 2 1 history traits such as size, the ecology of the organism, and kin selection, may together play a 2 2hidden, yet integral role in shaping senescence outcomes. 2 3 2 4 3 Main 2 5