A new study blends the fields of paleoanthropology and diving physiology to reconsider a long-standing question: did endurance running or endurance diving come first in human evolution? Researchers Josephine Joordens and Erika Schagatay propose a "dive first, run later" hypothesis, which challenges the widely accepted endurance running hypothesis. Rather than focusing solely on land-based foraging, they argue that both endurance running and diving should be considered when examining the evolution of human stamina. This perspective offers a more comprehensive view of how early humans adapted to their environments. Humans are unique among primates in their ability to perform both endurance running and endurance diving. Modern examples, such as the Ama divers of Japan and the Sama-Bajau of Southeast Asia, show that humans can repeatedly dive for hours to hunt fish and gather shellfish. This type of diving, known as "endurance diving," involves making multiple dives with short breathing pauses to maximize time underwater. For example, Japanese Ama divers can make up to 50 dives per shift, spending nearly half their time submerged. The endurance running hypothesis, introduced in 2004 by researchers Dennis Bramble and Daniel Lieberman, has long been the leading explanation for the evolution of human traits like long legs and efficient cooling systems. However, Joordens and Schagatay do not reject this idea entirely. Instead, they argue that it may be incomplete. Their "dive first, run later" hypothesis suggests that the evolution of human stamina could have been influenced by both running and diving, offering a broader explanation for the diverse foraging strategies seen in today's hunter-gatherers. Endurance diving requires strong respiratory and cardiovascular systems, as well as the ability to manage breathing and oxygen conservation efficiently. The researchers propose that these diving-related skills may have laid the groundwork for the cardiorespiratory stamina needed for long-distance running. They also note a connection between breath-hold diving and high-altitude tolerance, both of which involve adapting to low oxygen levels. Traits like a large spleen, which helps increase blood oxygen levels, are beneficial in both diving and high-altitude environments. The study also revisits the high bone mass found in many hominin fossils, which is often linked to terrestrial activity. Joordens and Schagatay suggest that this increased bone density may have evolved to aid in buoyancy control during shallow diving rather than from strenuous land-based tasks. This could explain why some early Homo populations, such as the earliest Homo erectus from Java, may have included underwater foraging in their lifestyle, while others with lighter builds may have focused more on running and land-based hunting. The researchers also reference recent DNA modeling that suggests all modern humans evolved from the merging of two ancient populations. They propose that the exceptional cardiorespiratory abilities of modern humans, including both diving and running, may have come from the combination of populations specialized in these different activities. This diversity in physical capabilities may have helped early humans adapt to various environments and contributed to the widespread success of our species. Joordens and Schagatay emphasize the need to integrate the hominin fossil record with human physiology research to evaluate their "dive first, run later" hypothesis alongside the endurance running hypothesis. They suggest future studies could explore the evolution of the external human nose, which may be an important feature for shallow diving without water entering the nasal passages. A combined approach examining both land and water environments in the fossil record may reveal new insights into human evolution that have been overlooked.