Evolutionary Trend in Hominidae | Geology Optional Notes for UPSC PDF Download

Evolutionary Trend In Hominidae

• Bipedal Locomotion with Erect Posture:
  • Humans adopted walking on two feet with straightened knees and flattened feet for efficient mobility.
• Development of Erect Posture:
  • The human spine evolved an S-shaped curvature, allowing for a fully upright posture.
• Changes in Pelvic Structure:
  • The pelvic girdle widened, and the position of acetabulum changed to support bipedalism.
• Forelimb Adaptations:
  • Arms were freed for manipulation, with opposable thumbs aiding in grasping objects.
• Loss of Body Hair:
  • Humans lost body hair, except on the soles of the feet, palms, and some parts of the face.
• Facial Changes:
  • Facial structure evolved towards an orthognathous face with reduced jaw size and teeth.
  • Eyebrow ridges and simian shelf decreased in size.
• Lower Limb Modifications:
  • The big toe lost its opposable nature, aiding in efficient walking.
• Cranial Capacity Increase:
  • The human brain expanded in size, with a high brain-to-body weight ratio, enabling advanced cognitive abilities.
• Speech and Memory Development:
  • Humans developed articulate speech and enhanced memory capabilities.

Pre-Primate Ancestry

• Sarcopterigian fishes (Lobe-finned fishes) evolved into Labyrinthodont amphibians.
• Labyrinthodont amphibians transitioned into Cotylosaur reptiles.
• Cotylosaur reptiles further evolved into Therapsid reptiles.
• Therapsid reptiles gave rise to Primitive mammals.
• Primitive mammals then led to Jurassic pantotheres.
• Finally, Jurassic pantotheres developed into Mammals of Order Insectivora.

Primate Ancestry

• The earliest primates emerged during the late Cretaceous period within the placental mammals group.
• These early primates likely shared close similarities with the insectivores from which they originated.

Closest Relatives of Humans

• Chimpanzee and Gorilla are considered the closest relatives of humans based on various similarities.

Structural Similarities

• Both Chimpanzees and Gorillas exhibit more or less bipedal locomotion.
• They have a semi-erect or erect posture.
• Absence of a tail is a common characteristic in both species.
• They possess fewer lumbar vertebrae compared to other primates.
• Presence of brow ridges is observed in both species.
• Grasping pollex (thumb) is a shared feature.
• Females of both species experience menstrual cycles.

Serological Similarity

• The AB blood group is found in both apes and humans.

Biochemical Similarities

• Analysis of hemoglobin shows a slight difference in one amino acid between chimpanzees and humans.
• Protein homology is highest between humans and chimpanzees, followed by gorillas and humans, and least between humans and Asiatic apes.

Cytological and Molecular Homology

• The fossil record of living African apes is unknown, so comparisons rely on cytological and molecular studies.
• Molecular studies suggest gorillas diverged from a common ancestor with chimpanzees and humans around 8-10 million years ago.
• Chimpanzees and humans diverged from a common ancestor approximately 6 million years ago.

Differences between Apes and Humans

• Similar DNA Content: In diploid cells, the DNA content between apes and humans is comparable.
• Genetic Similarity: When non-repeated DNA sequences of chimpanzees and humans are compared, they exhibit a striking 98% similarity in hybridization.
• Chromosomal Banding Patterns: Examination of the banding pattern between the chromosomes of apes and humans reveals that most human chromosomes closely resemble those of apes, with only slight differences in chromosomes 3 and 6.
• Chromosomal Count: Human beings possess 46 chromosomes, whereas apes typically have 48 chromosomes.

Despite these genetic similarities, anatomical distinctions between apes and humans have led to their classification into different families.

Differences Between Humans and Apes

• Humans have an erect, bipedal posture due to the "S"-shaped curvature of the vertebral column and broadened pelvis. In contrast, apes have a semi-erect posture with an elongated pelvis.
• Apes have rectangular or "U" shaped jaws, while human jaws are bow-shaped.
• Apes possess larger incisors, a "U"-shaped dental arch, and lower premolars with cutting edges. Humans have a smoothly rounded dental arch and smaller incisors.
• Apes exhibit a simian gap between incisors and canines, which is absent in humans.
• The palate of humans is arched, whereas in apes, it is flatter between the rows of cheek teeth.
• Apes have longer forelimbs compared to humans, whose legs are larger and stronger than their arms.
• Apes have a thick growth of body hair, while humans have significantly reduced body hair and a higher number of sweat glands.
• Gorillas have brow ridges and large crests on their skulls for muscle attachment, features not found in humans.
• The human brain is larger and more complex than that of apes, with enlarged parietal, frontal, and temporal areas of the cortex.
• Humans have a long neck and the foramen magnum positioned at the rear of the brain-case, whereas apes have a short neck with the foramen magnum position more ventral.

3. Ancestry of Humans (Early Evolution of Apes and Hominids)

• The origin of Hominidae, the family that includes humans, is a topic of debate among researchers.
• Some scientists believe that Aegyptopithecus, an ancient ape with a small brain, tail, and features suited for life in trees, might be the earliest known ape. It possibly evolved into the genus Dryopithecus during the Miocene era.
• Dryopithecus, also known as Proconsul, lived an arboreal life in Africa, Europe, and Asia around 15 to 25 million years ago. Fossils of Aegyptopithecus were discovered in Egypt, while Dryopithecus fossils were found in Kenya.
• Dryopithecus had a skull lighter than that of modern humans, lacking brow ridges. Its dental structure resembled a mix of ape and human features, with small incisors, large canines, and molars with a distinct Y-pattern arrangement.
• This group of scientists suggests that Eocene prosimians (like Lemurs and Tarsiers) could have evolved into Parapithecus during the Oligocene era, potentially serving as ancestors to both Old World monkeys and apes.

These summarized notes provide a simplified understanding of the early evolutionary history of apes and hominids, highlighting key developments and ancestral relationships in a concise and organized manner.

Earliest Hominids

• Ramapithecus (= Shivapithecus) existed in the late Miocene and early Pliocene, approximately 14-8 million years ago in the Siwalik Hills of India and Pakistan.
• Related fossils found in Maboko Island and Fort Ternan in Kenya, Hungary, and Greece are known as Kenyapithecus.
• These hominids are considered to be the closest ape-like forms to the ancestors of the Hominidae family.
• Fossil features suggest that they walked on two legs and had dental characteristics resembling those of modern humans.
• It is believed that they inhabited both forested areas and open lands based on their fossil features.

Ardipithecus Ramidus

• Ardipithecus Ramidus:
  • Discovery: Fragments with four teeth of a new fossil hominid species, dating back as far as 4.4 million years, were found in Ethiopia.
  • Previous Name: Formerly known as Australopithecus ramidus.
  • Characteristics:
    • Bipedalism: Ardipithecus Ramidus displayed bipedal characteristics, indicating the ability to walk on two feet.
    • Human-like Arms: The species had arms that were more human-like compared to other primates.
    • Incisiform Canines: These hominids had incisiform canines, suggesting dietary adaptations or social behaviors.
  • Significance: Considered as a potential ancestor, Ardipithecus Ramidus is thought to be the first ape-like hominid known to have existed.