Hominin Evolution

Humans share an extinct common ancestor with primates

Primate features Opposable thumbs – thumbs can touch fingers. This allows for grasping objects and climbing Large forward-facing eyes and flattened face to allow for stereoscopic (3D) vision (overlapping fields of vision)

Primate features Supra-orbital ridge (protective bony ridge at top of eye socket) Brachiation – shoulder is flexible due to ball and socket joint and can rotate arm behind body (good to swing between branches) Supraorbital ridge Brachiation in gibbons (a type of primate)

Primate features Large cranial capacity (brain case) compared to body size

Terms…… Hominid (Hominoid) – the group consisting of all modern and extinct Great Apes (that is, modern humans, chimpanzees, gorillas and orang-utans plus all their immediate ancestors). Hominin – the group consisting of modern humans, extinct human species and all our immediate ancestors (including members of the genera Homo, Australopithecus, Paranthropus and Ard ipithecus).

Hominin Cladogram Note: Chimpanzees are not part of the Hominins

Bipedalism (walking upright on 2 legs) Bipedalism distinguishes hominins from the other primates Advantages of bipedalism (learn these!) Leaves hands free to carry and make items (cultural evolution), hunt and forage Able to see over rocks and other obstacles Can regulate body temperature better (less surface area exposed to the sun) Utilises less energy to get around….extra energy can be used by the brain (belief that increased cranial capacity in hominins is linked to bipedalism) Fossil evidence shows that upright posture occurred before increases in cranial capacity (brain size)

Bipedalism and thermoregulation

Evidence of bipedalism in fossils Position of foramen magnum (hole in skull where the vertebrae join it) is more central

Evidence of bipedalism in fossils

S-shaped spine helps support weight and maintain centre of gravity Shorter, wider pelvis indicates that the muscles attached to it are at such an angle that they are involved in walking upright with gravity centred through the middle of the organism Femur angled inwards ensures weight is kept central and directed towards the knees.

Evidence of bipedalism Forward facing big toe – centres weight Non-hominin primates have their larger digit pointing outwards – good for grasping branches

Before the genus Homo there was Australopithecus

Australopithecus

Our ancestors come from Gracile Australopithecus A robust body is heavy in skeletal structure and muscle (cannot move fast) A gracile body is lighter and more slender (faster) Fossil evidence suggests that the robust Australopithecines line became extinct. Therefore modern human ancestors (the Homo genus) came from the gracile Australopithecus lineage.

The genus Homo Reduced canines from chimpanzee to genus Homo More parabolic (curved) jaw shape Loss of gap (diastema) between canines and premolars Less molars Suggests less tough fibre and less reliance on meat in genus Homo

The genus Homo Larger cranial capacity (don’t write “larger brain size” as the brain is no longer present to directly observe)

The genus Homo With an increase in cranial capacity, more nerves needed to travel between the brain and the body The hole in the centre of the vertebrae increased in size from Australopithecus to Homo, allowing extra space for the extra nerves (this feature was selected for) Central canal

The genus Homo Flatter faces and use of tools (cultural evolution) in genus Homo

Transitional fossils between Australopithecus and Homo genus

Zygomatic arch and sagittal crest is smaller in genus Homo Zygomatic arch in a herbivore is larger to accommodate more jaw muscles for more chewing

The sagittal crest is an attachment point for muscles involved in chewing. If it is larger, there is more fiber in the organism’s diet. Sagittal crest Zygomatic arch

Order the following from closest human to least closest human ancestor

Phylogenic Tree of Human Ancestors Not a direct ancestor of modern humans First to use fire and leave Africa First to use tools Robust (not gracile) Able to walk upright

Separate species or interbreeding? Two theories: Multiregional hypothesis -modern humans evolved from different tribes of Homo erectus and evolved separately 2) Replacement hypothesis (Out of Africa hypothesis) -modern humans came from one tribe of Homo erectus and replaced all other tribes. -Neanderthals, therefore, were evolutionary dead ends

Out of Africa Hypothesis is supported by more evidence Fossil evidence (transitional forms mostly found in Africa) Mitochondrial DNA (mtDNA variation is greater in African populations compared to other worldwide populations. Shows that African populations have been around longer and therefore more time for more accumulations in mutations to occur). Y chromosome DNA variations (as per mtDNA but inherited paternally)

mtDNA haplogroups and Out of Africa Theory Oldest haplogroup is L, followed by M

Why do we all look different? Different regions have different selection pressures This results in different traits being selected for (e.g amount of pigmentation in skin) Skin pigmentation correlates with latitude because it serves as a defense against UV radiation.

Cultural Evolution Learned behaviours transmitted throughout generations (development of customs and languages) e.g. music, art, pottery, writing, jewellery

Technological Evolution Changes in technology which give humans more control over their environment e.g. development of tools, agriculture, medicine, space travel, communication devices