Periodic catastrophes over human evolutionary history are necessary to explain the forager population paradox

Abstract

The rapid growth of contemporary human foragers and steady decline of chimpanzees represent puzzling population paradoxes, as any species must exhibit near-stationary growth over much of their evolutionary history. We evaluate the conditions favoring zero population growth (ZPG) among 10 small-scale subsistence human populations and five wild chimpanzee groups according to four demographic scenarios: altered mean vital rates (i.e., fertility and mortality), vital rate stochasticity, vital rate covariance, and periodic catastrophes. Among most human populations, changing mean fertility or survivorship alone requires unprecedented alterations. Stochastic variance and covariance would similarly require major adjustment to achieve ZPG in most populations. Crashes could maintain ZPG in slow-growing populations but must be frequent and severe in fast-growing populations—more extreme than observed in the ethnographic record. A combination of vital rate alteration with catastrophes is the most realistic solution to the forager population paradox. ZPG in declining chimpanzees is more readily obtainable through reducing mortality and altering covariance. While some human populations may have hovered near ZPG under harsher conditions (e.g., violence or food shortage), modernHomo sapienswere equipped with the potential to rapidly colonize new habitats and likely experienced population fluctuations and local extinctions over evolutionary history.

Human uniqueness illustrated by life history diversity among small-scale societies and chimpanzees

“…If adult mortality rather than fertility limits chimpanzees' reproductive potential, then human and chimpanzee life histories would be even more similar under conditions of low adult mortality. IBIs in chimpanzees are lower when infants die early, and so average IBI is affected by early life survival [14,35]. Unlike humans, lower juvenile mortality would only lengthen chimpanzee IBIs, and therefore reduce lifetime fertility.…”

“…Though imperfect representatives of the past, the differences we observe within and between species nonetheless offer a unique opportunity to learn about the forces shaping human life history. Previous findings suggest that alternative demographic routes to human persistence are reflected in life history adaptations that maintain the potential for high intrinsic growth rates and allow recovery from periodic population crashes [14]. The Ju/'hoansi !Kung, Hiwi and Gainj, with low and delayed fertility, hover near-stationarity and are on the slower side of a life history continuum, while the Tsimane and Yanomamo are on the faster side with early and high fertility driving rapid population growth, perhaps in response to post-colonization recovery.…”

“…As with any pair of lineages, we are confronted with questions about the conditions under which human and chimpanzee life histories diverged, since they may have faced very different selection pressures over evolutionary time since their divergence from a common ancestor. Also, because we are sampling human societies that survived contact, our sample may over-represent growing populations especially since these short-term data may have captured transient growth periods in population cycles with rapid declines and prolonged recovery [14].…”

“…Whereas many contemporary small-scale societies are growing rapidly [11], documented chimpanzee populations are typically shrinking, though favorable conditions promote increase in some wild groups [12,13]. With the largest high-quality dataset assembled to date on fertility and mortality among human subsistence societies and chimpanzees [14], we employ life table response experiments [LTREs,15] to quantify the importance of particular age classes for driving population growth and identify the vital rates that best explain the divergence of human and chimpanzee life histories.…”

Marine resource abundance drove pre-agricultural population increase in Stone Age Scandinavia

“…Holocene population trends across the world have been extensively studied with the development of 14 C dates as data methods 3,4,7,17,18 . This approach has shown the global population increase to be very gradual until recently, with near to zero population growth (0.04%) throughout much of history 18,19 , though punctuated by distinct boom and bust phases 7 , with catastrophic periods identified as a key factor (along with altered mean vital rates) constraining population growth 18 . The repeatable, but regionally variable boom and bust pattern has generated considerable debate with regard to the possible drivers of human population responses to a multitude of factors and stressors, including, socio-economic status, disease, war, environmental and climatic change as well as resource availability 3,14,[20][21][22] .…”