Human Evolution (Day 4)

Did we evolve from chimps?

No, but we evolved from a common ancestor with them

Hominids

Human lineage, bonobos, chimps

Hominins

• Humans and their extinct ancestors

• Extinct ancestors give an opportunity to place derived traits along lineage leading to us!

Hypothesis and evidence of relationship of chimpanzees and humans

• Humans didn’t evolve from modern apes but both chimpanzees and humans evolved from a common ancestor

• Shared morphological(physical) traits is one piece of evidence

• Fossil evidence (extremely hard to find)

• DNA evidence as difference between human and chimps is 0.9%, humans and gorillas is 1.4%, human and orangutangs is 2.9%

Unique human traits

• Bipedalism

• Brain size expansion

• Shorter gut

• Hair changes

• Bigger newborns and long development

• Handedness (left or right handed)

• Emotional weeping

• New behavior: symbolic thinking, personal ornaments, art, trade

Bipedalism

• Walking on 2 legs

• Frees forelimbs to manipulate objects/carry items

• Walking to on two legs saves more energy

• Maybe adaptation to living on open grass land?

• Came before brain expansion

Brain size expansion

• Linked to climate fluctuations, hypothesized that greater cognitive ability was beneficial in unpredictable environments

• Cons are challenging birth and long juvenile period which often requires parental care, brains are energetically expensive, hypothesized that loss of body hair related to thermoregulation of our “hot” brains (scalp hair kept as barrier to sun)

Gut size and diet

• Increased reliance on meat bc easier to digest

• Digestion and gut tissue are energetically expensive

• Cooking further eased digestion

• Changes in diet that make shorter guts may have freed resources for brain development

Australopithecus afarensis

• 3.9-3 mya

• Already fully bipedal

• Femur to humerus ration more similar to chimps (long arms, short legs)

• Small brains

Paranthropus

• Evolutionary side branch of Australopithecus afarensis

• Reconstruction of inner ear bones suggest that they could hear high-frequency constants

When did homo genus evolve?

2.5 mya

Homo habilis

• “Handy Man”

• Larger brain

• Tools!!

• 2.5 mya

Homo erectus

• 1.8 mya

• Bigger brains that homo habilis

• Cooler tools

• Fire!!

• Extinct 250k ya

Homo neanderthalensis

• Appeared by 150,000 ya

• Europe, Middle-East, East to Uzbekistan

• Peak 70-30,000 ya

• Die out about 27,000 ya

• Art? Newly discovered jewelry

• Religion/Ritual?

• Occasional simple burials

Homo floresiensis

• Extinct by ~17000 ya

• Originally thought to be a juvenile skull

• Insular dwarfism from what ancestral species?

Denisovans

• Argued if Homo species or subspecies of Homo sapiens (humans)

• Harbin cranium (a well preserved skeleton head) identified as being part of this group (newly named as Homo longi)

Homo sapiens

• Anatomically modern humans appear about 100k ya

• Africa first

• Much later elsewhere (Asia is 60k ya, Europe is 40k ya, New World is 11k ya, Australia is 56k ya)

Migration of humans

Once leaving Africa about 115k ya, they split into 3 different directions:

• Towards Europe

• Towards Australia

• Towards South America (got to North around 18k ya and South around 13k ya)

Climate, amount of UV, and historical diet and more effects

• Darker skin when more UV radiation

• Vitamin-D deficiency (common farther away from equator)

• Eye color (lighter w/ limited light)

• Cranial capacity (slightly larger in colder regions)

• Lactose intolerance (mutation most common in Europe)

• Malaria resistance (sickle cell)

Human Mate Choice effects

• Facial symmetry is an attractive trait due to its association with good health and resistance to developmental stress

• Good female waist-to-hip ration may signal fertility and lack of current pregnancy

• Males w/ low waist to chest ration may have lower body fat and higher levels of testosterone

• Hetero people identifying as women have preference with beard and lower voices

Reduced selective pressure effects

• Glasses emerging caused the pressure for good sight to go down

• Reduction in jaw size over evolution meant crowding of teeth, but now people get wisdom teeth removed to fix that

• Childbirth was a selective force until people started to use C-sections

• Heavy smoking leads to Alzhemier’s and cardiovascular issues despite interventions

• Sexual selection still at play