Why does where your ancestors lived matter so much?
The strongest broad explanation in the evidence is history and geography rather than climate. A study of skull measurements across human populations found that distance from sub-Saharan Africa, not climate, explained the pattern of physical diversity within populations [2]. In plain terms, groups that split off and moved farther from Africa earlier had more time to drift apart genetically, and that accumulated difference shows up in bodies and faces.
This is a statistical pattern across populations, not a rule about any individual. It describes why average differences exist between groups separated by long distances and time, not why any one person looks the way they do.
If groups differ, why can't you tell people apart by a single feature?
A detailed study of the foramen magnum—the hole at the base of the skull where the spinal cord passes—found that while there were significant average differences between sexes and between populations, those differences were small and swamped by large overlaps between the groups [1]. In other words, the average male skull base and the average female skull base differ slightly, but most individuals fall in a shared middle range.
The same study suggests that this variability is likely driven by a mix of genetic, environmental, and cultural factors that are not yet fully clarified [1]. That is a statement about possible causes, not a settled account of how much each contributes.
How do sex and ageing change a face?
Sex and age are two everyday sources of variation that operate within every population. A study using 3D facial scans of adults examined how facial form differs between men and women and how it changes with age, treating sexual dimorphism as something to observe across the whole adult age range rather than at a single moment [4]. The practical point is that a face is not fixed: the same person's facial form shifts as they age, and the typical differences between men and women are part of that moving picture.
What do genes actually contribute?
Part of the answer lies not just in which genes you carry but in how strongly they are switched on. Promoters are stretches of DNA where gene reading begins, and they are hotspots for genetic variation linked to physical traits [5]. Some promoters are evolutionarily "volatile": they have been gained or lost over evolutionary time, and their presence is associated with more variation between individuals in how much a gene is expressed [5].
That said, the same study found that this molecular variation did not translate into a matching enrichment of physical traits at those loci, and much of the expression variation appeared to be selectively neutral—present but not consequential for appearance [5]. So volatile promoters are a plausible source of evolutionary innovation and phenotypic variation, but the evidence points to a large background of variation that does not visibly change the body.
Sources used in this answer
Two- and Three-Dimensional Osteometric Variability of the Foramen Magnum in Modern Humans
Average differences in foramen magnum size and shape between sexes and populations are small and heavily overlapped, and variability is likely driven by genetic, environmental, and cultural factors not yet fully clarified.
Distance from Africa, not climate, explains within-population phenotypic diversity in humans
Distance from sub-Saharan Africa, rather than climate, explains the pattern of within-population phenotypic diversity in humans based on skull measurements.
Who goes there? Approaches to mapping facial appearance diversity
Location-dependent generative models can map how facial appearance variability differs across countries, such as comparing Taiwan and Germany.
Sexual dimorphism of facial appearance in ageing human adults: a cross-sectional study
A 3D scan study of adults examines how facial form varies with age and differs between sexes across the adult age range.
The contribution of evolutionarily volatile promoters to molecular phenotypes and human trait variation
Evolutionarily volatile promoters are associated with increased between-individual gene expression variance, but this largely does not translate into enriched physical trait variation.