New research has tracked the body size of animals across history and found that domestic animals in Mediterranean France grew during the early Middle Ages while wild species shrank, most likely due to human pressures.
For most of history, both animal groups followed similar body-size patterns. The findings demonstrate the increasing role humans and human activities have on these changes to animal evolution.
"These findings highlight the dynamic and interwoven roles of environmental and anthropogenic factors in shaping animal morphological evolution, illustrating the growing impact of human activities on wild populations," the authors write in the study published in the Proceedings of the National Academy of Sciences.
The researchers conducted the study by analysing a total of 225,780 bones from the past 8,000 years. The bones were collected from 311 archaeological sites in Mediterranean France.
The team focused the analysis on wild animals like red deer, foxes and rabbits, as well as domestic animals like sheep, goats, cattle, pigs and chickens. They also took into consideration factors such as the climate and human activity.
They used a type of statistical modelling called 'regression analysis' to detect body size variation through history. The results showed animal body sizes changed over time in a non-linear way, which the researchers separated into 4 key phases.
The first phase, 6,000 to 2,000 BCE, occurred during the Neolithic Period when humans began to transition away from hunter-gather lifestyles and towards agricultural societies.
"During the Neolithic Period, the body size of all species, wild and domestic, was highly correlated and consistently declined through time, suggesting a common response to anthropic [human] and environmental influences," write the authors.
These results are consistent with the current understanding that since environmental and anthropogenic pressures varied over the years, so too would body size.
During the second phase, which spanned from the Bronze Age to the Roman Principate (2,000 BCE to 300 CE), domestic species grew in size due to greater access to forest cover and better managed agricultural areas.
Similarly, the size of wild animals also rose, which the authors suggest may be due to humans opening the landscape to grasslands that wild animals like red foxes thrive in.
From the Late Antiquity to the early Middle Ages (300 to 1,000 CE), wild and domestic animals' body sizes declined due in part to the fall of the Western Roman Empire and associated farming, and to an 'erosive crisis' which likely weakened the habitats of wild species.
However, it was during the fourth phase (1,000 to 2,000 CE) that the researchers noticed a considerable divergence in body sizes between wild and domestic animals. In particular, there was a shift in animal body size from the Middle Ages onward.
"In the last millennium, these trajectories diverged as human influence intensified, promoting unprecedented increases in domestic species body size, while wild species exhibited a trend toward decreased size," write the authors.
The authors suggest that the increase in size of domestic animals is likely the result of an increase in human activities like livestock farming and selective breeding. In contrast, the steady decline in wild animal size over time is attributed to increased hunting and habitat destruction also caused by humans.
"Understanding the long-term interplay between human societies, environmental changes, and animal morphology is a fundamental question in evolutionary biology," write the authors. "While human activity is widely recognised as a powerful evolutionary force, our results demonstrate that natural selection prevailed as an evolutionary force on domestic animal morphology until the last millennium."