8 Hominins

What is a Hominin?

  • A hominin includes humans and all of our ancestors. The hominin line split from other apes approximately 7-6 million years ago, towards the end of the Miocene epoch. It's important to understand not just the definition, but also the traits that distinguish hominins.

🧠 Early Hominin Trends

The image above shows the trend of early hominins, with an increase in brain size linked to changes in skull shape. The following trends characterize the evolution of early hominins:

  • Decreased Prognathism, Increased Orthognathism:

    • Prognathism refers to the extent to which the maxilla (upper jaw) sticks out.

    • Orthognathism refers to the extent to which the mandible (lower jaw) sticks out.

    • Hominin evolution sees a decrease in prognathism and an increase in orthognathism, contributing to the development of a chin in Homo sapiens.

  • Post-orbital Constriction: Tightening behind the eye orbits is related to the size and position of mastication (chewing) muscles.

  • Dental Arcade Changes: The tooth row shape changes from parallel to parabolic.

  • Nonhoning Chewing: The diastema (gap between teeth) and sectorial premolar disappear over time, associated with a decrease in canine size, and dental enamel becomes thicker.

🚶 Anatomical Features of Bipedalism

Bipedalism is the defining characteristic of hominins. Key anatomical features that denote the shift to bipedalism include:

  • Anterior Foramen Magnum: The foramen magnum, the hole at the base of the skull through which the spinal cord passes, is located more anteriorly (towards the front) in bipeds.The image demonstrates the anterior foramen magnum in homo sapiens.

  • S-Shaped Vertebral Column: The S-shape allows the weight of the trunk to be centered over the pelvis. Lumbar curvature helps align the center of gravity. In contrast, apes have a C-shaped vertebral column. Gorillas also have one more thoracic vertebra and one less lumbar vertebra to account for their C-shape.

  • Bowl-Shaped Pelvis: This shape, along with associated gluteal muscles, supports upright posture and efficient bipedal movement.

  • Bicondylar Angle: This is the angle of the femur relative to the knee. Femora are angled in humans, giving a knock-kneed appearance and providing support for the center of gravity. Babies develop this angle as they learn to walk. When apes walk upright, they bend their knees.

  • Leg Length: Humans have longer legs compared to their arm length, which allows for a more efficient bipedal gait.

  • Adducted Big Toe: The big toe is in line with the other toes, which supports weight better for bipedal locomotion.

  • Longitudinal Foot Arch: This arch acts as a shock absorber and provides leverage for walking and running. Force is distributed differently in habitual bipeds: heel strike, roll through to the ball of the foot, and out through the big toe.

🤔 Why Did Hominins Emerge?

Several hypotheses attempt to explain why hominins evolved:

  • Patchy Forest Hypothesis: As forests became smaller at the end of the Miocene, quadrupedal motion was inefficient. Bipedalism evolved as a way to move faster between areas with food and see over grasses. Walking upright also freed hands for grabbing/carrying food.

  • Provisioning Hypothesis (aka Male Provisioning): Males travel for food to allow females to stay in place and raise more babies. Monogamous male/female pairs would allow for protection of young and cooperation. Males needed free hands to carry food back. Relatively low levels of sexual dimorphism in early hominins supports this idea of cooperation instead of competition.

  • Thermoregulation: Standing upright allows for less exposure to solar radiation, potentially leading to the loss of body hair.

  • Endurance Running: This could be a by-product of long-distance hunting and likely evolved after bipedalism. Leg, gluteus muscles, and trunk muscles all evolved for better structure and support for bipedal locomotion (and maybe running).

âš  Pitfalls of Bipedalism

It's important to consider the downsides of bipedalism:

  • Exposure to predators: Early hominins were likely the prey.

  • Slipped intervertebral discs, arthritis, varicose veins.

  • Exposed abdomen.

  • Difficult to give birth.

  • Hemorrhoids.

  • Hip/knee/foot injuries inhibit movement.

No single hypothesis explains the origin of bipedalism sufficiently. Bipedalism was the starting point for additional developments, such as larger brains.