Humans have a very unusual life-history: they are born completely helpless, take a prolonged period to mature, have surprisingly short intervals between child births, show complete cessation of reproduction long before death (the menopause) and tend to die of metabolic diseases (heart disease, diabetes) that other species do not suffer. Understanding how all of these unusual traits evolved demand studying patterns of natural selection and evolution in human populations, but collecting data from across the whole life of contemporary populations is impractical.
My post-doctoral work with Virpi Lummaa used data collected from pre-industrial Finnish chruch records to test hypotheses related to the effects of environmental variation on human life-history evolution, particularly how conditions experienced in early life may lead to the development of metabolic diseases. This is a branch of research known as the ‘development origins of health and disease (DOHAD).
In particular, we tested the predictive adaptive response (PAR) hypothesis. The hypothesis predicts that maternal signals allow organisms to develop in such a way to deal best with the environment they may experience in the future. For example, if mothers are in a high-food environment, the offspring should develop a ‘fast’ metabolism, or if the mother experienced food deprivation, the offspring should develop in such a way as to make the most of every scrap of energy they gain. However, a mismatch is predicted to lead to metabolic disease and low survival and reproductive success.
We tested this hypothesis using data collected during a severe famine, which was associated with catastrophically low crop yields and high death rates during the years 1866-68 (see Figure below). The PAR hypothesis would predict higher survival and reproductive success during the famine in those which experienced low crop yields in early life: higher fitness in individuals experiencing matching conditions in early and later life. Instead, we found the opposite: low crop yields around birth led to lower survival and reproductive success during the famine, suggesting that, contrary to the hypothesis, low nutrition in early life did not enhance fitness during a period of low nutrition in later life.
My current work focuses on how exposure to infectious diseases during early life is associated with later survival and reproductive success. This is one of the hypotheses which seeks to explain recent improvements in human lifespan.