Travel Between Dissimilar Environments

Of all primate species, humans actively travel and readily adapt to new environments within an individual’s lifespan. While not all individuals within a given population may have the material resources or behavioral inclination leave their homebase, all humans maintain the ability to inhabit and/or move between nearly every biome on earth. Uniquely isolated human populations possess heritible traits best suited for their particular enviroments (i.e., milk drinking Western Europeans to pastoral areas; Tibetans' physiological adaptations to the high altitude Himalayan plateau). While these population differences in ability to adapt to changes in environment or diet may impact the areas to which some human populations may travel, the majority of humans maintain the ability to adapt (immediately or gradually) to inhospitable environments to which they are not genetically adapted.

All species are capable of varying levels of ‘phenotypic plasticity,' in that a single genome can produce different phenotypes in response to environmental variations. Since the majority of environmental factors do not alter DNA or promote genetic mutation, plastic responses to environmental changes can lead to altered patterns of selection on related traits that do exhibit heritable variation. Phenotypic adaptations within an individual's lifespan, or even an individual's offspring may also be due to epigenetic changes (considered mostly non-heritable other than transgenerational epigenetic changes). Due to the breadth of human expansion and actively employed ability to travel between environments, humans may be capable of a uniquely diverse breadth of phenotypic plasticities allowing adaptations to varying climates within an individual's lifespan.

While other ape habitats encompass multiple ecosystems and may historically have occupied diverse environments, only modern humans display the ability to adapt to environments ranging from desert to arctic within any individual's lifespan. Some of these human adaptations are certainly linked to human technological advances (i.e., residency at the South Pole is only made possible by highly developed shelter and clothing), however, physiologic plasticity to variable environments may not be present in other ape species. Further, only humans regularly travel between dramatically dissimilar environs within very short timeframes (given human cultural advances, the only limitations to human travel and adaptation remain technology). 

This uniquely employed physiologic plasticity may have been a driving factor allowing modern humans to become so widespread and populous. This is likely since small population sizes are limited in the rate of adaptive evolution whereas a population that increases in size may allow for rapid adaptive changes. This theory has been recently supported by genomic surveys into human genetic variability which identify an acceleration of recent positive selection consistent with recent population growth. These growths in demography are potentially from changes in human culture and ecology allowing population expansion.

Adaptation to upright walking allows more efficient travel; thus this behavior may have granted humans the unique ability to inhabit and travel through new climates.

**********

On the origin of uniquely-human niche breadth:

Humans exhibit extraordinary phenotypic plasticity within individuals, a trait which allows large populations of the same species to exist in climates separated both by great distance and differing environment conditions. It is likely that large growth in population size increased the rate and effectiveness of adaptive responses; small population sizes are limited in the rate of adaptive evolution whereas a population that increases in size may allow for rapid adaptive changes. This theory has been recently supported by genomic surveys into human genetic variability which identify an acceleration of recent positive selection consistent with recent population growth. These growths in demography are potentially from changes in human culture (i.e., technological advances) and ecology (i.e., adaptable diet) allowing population expansion.