Topic 4- Hominid evo Flashcards Preview

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Molecular evidence shows the following times for when different lineages split off from the hominoid lineage

• Chimpanzee & bonobo lineage: at least 7-8 million years ago.
• Gorilla lineage: 9-10 million years ago.
• Orangutan lineage: 12-15 million years ago.
• The last common ancestor for the human
lineage and great apes was about 15 million years ago.


Langergrager et al., PNAS,
109(39):pp.15716-15721, 9/25/2012

• Genetic studies estimate that the hominin-chimpanzee split is at least 7-8 million years ago


Hominids = humans and their closest extinct ancestors

• Sahelanthropus tchadensis
• Ardipithecus ramidus (Ardi) & A. kadabba
• Australopithecus afarensis (Lucy) & A. africanus
• Homo habilis
• Homo erectus (in Africa & Asia) and its offshoots Homo heidelbergensis (European clade) and Homo rhodesiensis (African clade)
• Homo neanderthalensis • Homo sapiens


Sahelanthropus tchadensis

• 7 million years old specimen of Sahelanthropus tchadensis found near Lake Chad in Central Africa.
• Brain size is estimated to be 320 - 380 cm3 which is similar to the size of extant chimpanzee brains.
• Most significant specimen found is a partial cranium.
• May represent the earliest known hominin/hominid.
-site of origin near Lake Chad


Hominid evolution

• 4.4 million years old: Ardipithecus ramidus “Ardi”from Afar region of Ethiopia.
• 3.2 million years old: Australopithecus afarensis “Lucy”from Ethiopia.
• Partial bipedalism appears in early hominins/hominids, with Ardipithecus ramidus both a tree climber and a bipedal walker and the Australopithecus afarensis
being clearly bipedal.
• Bipedalism preceded the appearance of the dramatically larger neocortex.
• Changes in dentition and jaw shape also evolved during hominin/hominid evolution.


Ardipithecus ramidus

• Female (4 ft tall) who lived 4.4 million years ago in the Afar region of what is now Ethiopia.
• 125 pieces of skeleton, including most of skull and teeth.
• Discovered by Professor Tim White in the Department of Integrative Biology.
• In Science October 2, 2009 there were 11 concurrent articles giving a comprehensive
description of Ardi.
• In Science December 18, 2009 the Ardipithecus ramidus discovery was celebrated as scientific breakthrough of the yea
•Ardipithecus ramidus represents a species that climbed trees, but also could walk on two legs or on all four extremities, but walked on the palms of the hands instead of the knuckles of the hands like chimpanzees and gorillas.
• Small brain measuring 300-330 cm3 which is about the size of a modern bonobo or female common chimpanzee but, much smaller than Lucy (Australopithecus afarensis) whose brain was 400-550 cm3.
• Lucy lived 3.2 million years ago and was discovered just 47 km from the site where Ardi was discovered.
• Ardi’s and Lucy’s brains were both much smaller than the modern Homo sapiens brain which is 1350 cm3.


Ardipithecus ramidus
Skull, jaw, & teeth

• Muzzle juts out less than chimps.
• Both females and males lack the dagger-like upper canines which are present in male chimpanzees.
• Vegetarian.
• Base of skull is short from front to back as in upright walkers rather than elongated like in quadripedal apes.


Ardipithecus ramidus

• Upper blades of Ardi’s pelvis are shorter and broader than in extant apes, lowering her center of gravity so she could balance on one leg at a time while walking.
• Pelvis also contains a sciatic notch, a large opening that is a key trait with later hominins/hominids; sciatic nerve passes through this notch to enter the posterior thigh.
• Ardi’s pelvis enabled her to both climb trees and walk upright, making her a “facultative”biped.
• However, she did not walk as well as Lucy (Australopithecus afarensis) or Homo sapiens.


Ardipithecus ramidus
hands and feet

• Ardi’s wrist joints were less stiff than apes and the bones in her palms are short which indicates that she did not knuckle walk or swing through trees like chimpanzees.
• When walking on all four extremities, Ardi walked on the palms of the hands.
• Ardi’s long curving fingers and opposable big toe suggest that she grasped tree branches.
• Ardi has relatively flat feet that were less well equipped for walking than the more arched feet of Australopithecus (Lucy).


Ardipithecus ramidus Ecosystem

• An international research team collected over 150,000 specimens of fossil plant and animals from around the area where the Ardi bones were discovered and carefully dated these specimens to determine which were 4.4 million years old.
• With this information, they were able to reconstruct the ecosystem in which Ardi lived.
• The research team concluded that Ardi lived in an ancient floodplain covered with woodlands with trees of fig, palm, and hackberry, and co-existing with monkeys, kudu antelope, and peafowl.
• This discovery is counter to the presumption that upright bipedal walking in hominins/hominids emerged in populations living in open grasslands.


Australopithecus afarensis

• Bipedal with arched feet compared to flat feet of Ardipithecus ramidus.
• Sexually dimorphic.
• Weighed about 45 kg (100 pounds).
• Brain size 400-500 cm3, which is a slightly
larger brain relative to body size compared to the extant chimpanzee.
• Were likely mainly vegetarian and lived in a woodland habitat
• 3.2 million years old.
• Body height of Lucy estimated to be 3’6”
which is significantly shorter than other Australopithecus fossils.
• Pelvis and legs bones almost identical in function to modern humans.
• Pubic arch in females was over 90 degrees, similar to modern humans.
-not likely able to dance
• It is estimated that by 2.5 million years ago, Australopithecus species (e.g., Australopithecus afracanus) had emerged which had larger brains, smaller jaws, and smaller teeth which suggest a mainly vegetarian diet, but with possible inclusion of some animals in their diet.



• Lucy is the most famous specimen of Australopithecus afarensis and was discovered and described by Donald Johanson, Maurice Taich, and Tim White.


Evolutionary sequence & brain size

• 7-8 MYA:hominin/hominid lineage split from chimpanzee lineage.
• Pan troglodytes (chimpanzee) brain size today 300-400 cm3.
• Pan paniscus (bonobo) brain size today 300-400 cm3 .
• 7 MYA: Sahelanthropus tchadensis brain size 320-380 cm3.
• 4.4 MYA: Ardipithecus ramidus brain size 300-330 cm3.
• 3.2 MYA: Australopithecus afarenesis brain size 400-500
• 2.5 MYA: Homo habilis brain size 600 cm3.
• 1.8 MYA – 200,000 YA: Homo erectus brain size 680-1100
cm3 and its offshoot clades in Africa and Europe.
• 400,000 – 28,000 YA: Homo neanderthalensis brain size similar to Homo sapiens.
• 200,000 YA - present: Homo sapiens brain size 1350 cm3


Summarized Homo

Homo habilis 2.5 - 1.6 million years ago (brain size 600 cm3) lived in Africa.
Homo erectus (originated in Africa & migrated to Europe & Asia) 1.8 million - 200,000 years ago (brain size 680-1100 cm3) including its various offshoot clades which produced the following:
• Homo heidelbergensis (European clade offshoot of H. erectus) 500,000 - 350,000 years ago (brain size similar to
Homo sapiens) lived in Europe and likely lead to H. neanderthalensis.
• Homo rhodesiensis (African clade offshoot of H. erectus) 550,000 - 125,000 years ago (brain size 1000 - 1100 cm3) lived in Africa and likely lead to H. sapiens.
Homo neanderthalensis emerged 150,000 - 400,000 years ago and became extinct 28,000 years ago (brain size similar to Homo sapiens) lived in Europe.
Homo sapiens 200,000 years ago (brain size 1350 cm3) originated in Africa and now lives worldwide.


Proposed Homo evolution scenarios

• Homo heidelbergensis is likely European clade
offshoot of Homo erectus.
• H. rhodesiensis is likely African clade offshoot of
Homo erectus.
Proposed Homo evolution scenarios:
• H. erectus > H. heidelbergensis > H. neanderthalensis
• H. erectus > H. rhodesiensis > H. sapiens


Transition from the Australopithecus genus to the Homo genus

• Research by Tim White suggests that the transition from the Australopithecus
genus to the Homo genus occurred around 3.0-2.5 million years ago.
• Australopithecus garhi has been proposed to be the likely ancestor of Homo habilis because of the similarity between their face, jaws and teeth.


Homo habilis

• Around 2.5 – 1.8 million years ago the earliest known species of Homo (Homo habilis) lived with a body size up to 45 kg with a height from 3’6”to 4’3”tall and a brain size of up to 600 cm3.
• Evidence that Homo habilis made and used stone tools.
• Specimens of Homo habilis were first discovered in Tanzania in East Africa by Mary and Louis Leakey; later Tim White and Donald Johanson discovered additional specimens in Tanzania.


Homo erectus

• 1.8 million to 200,000 years ago.
• Adult height of (5 feet 2 inches) and brain size of 680-1100 cm3 • Originated in Africa 1.8 Mya
• Early Homo erectus includes Homo erectus ssp. ergaster
which was present 1.8-1.3 million years ago and represents
an early form of Homo erectus.
• By 1.8 Mya Homo erectus migrated through Europe to Georgia in Eurasia
• By 1.7 Mya Homo erectus migrated to Java in Indonesia. • By 1.4 Mya Homo erectus migrated to China. • Homo erectus appears to have had a prolonged childhood
and a life history pattern intermediate between that of modern
great apes and Homo sapiens.
• Skeletal remains of Homo erectus dating 1.8 Mya were found in Africa.
• “Turkana Boy”skeleton is dated at 1.3 million years old was found near Lake Turkana in Kenya and represented a 5’3 inch boy estimated to be 13 years old.
• “Peking (Beijing) Man”skeleton discovered in China is estimated to be 500,000 years old; other skeletal remains of Homo erectus dating back 1.4 million years have been found in China.
• “Java Man”skeleton discovered on island of Java, Indonesia dated at 1.7 million old.


Homo erectus and its offshoot clades IMPORTANT

Homo erectus (in Africa & Asia) 1.8 million -200,000 years ago (brain size 680-1100 cm3)
including its offshoot clades:
• Homo heidelbergensis (European clade offshoot of H. erectus) 500,000 - 350,000 years ago (brain size similar or slightly larger than Homo sapiens) likely produced H. neanderthalensis.
• Homo rhodesiensis (African clade offshoot of H. erectus) 550,000 - 125,000 years ago (brain size 1000 - 1100 cm3) likely produced H. sapiens


Proposed Homo evolution scenarios

H. erectus > H. heidelbergensis > H. neanderthalensis.


Homo heidelbergensis

• Homo erectus likely gave rise to the European clade Homo heidelbergensis
around 500,000 years ago.
• Homo heidelbergensis (European clade offshoot of H. erectus) 600,000 - 350,000 years ago (brain size similar to Homo sapiens).
• Homo heidelbergensis appears to have given rise to Homo neanderthalensis


Homo neanderthalensis

• Homo neanderthalensis evolved from H. heildelbergensis from 150,000 to 400,000 years ago and their distribution was restricted to Europe, the Middle East and western Asia.
• Populations of Homo neanderthalensis
were most robust between 100,000 to close to 28,000 years ago.
• Homo neanderthalensis was a different population than Homo sapiens.


Evidence of Homo sapiens interbreeding with Homo neanderthalensis Europe (Nature, vol 328, pp 710-722, 7 May 2010)

• Genetic studies of present-day Homo sapiens from lineages outside of sub-Saharan Africa indicates that on average 2.5% of the DNA is from Neanderthal.
• Studies showed that Neanderthals shared genetic variants with present-day humans (Homo sapiens)
in Europe and Asia but not with present-day humans (Homo sapiens) in sub-Saharan Africa.
• This can be explained by the fact that Neanderthal range did not extend into Africa.


Homo neanderthalensis

• Brains similar in size to modern Homo sapiens.
• Robust with relatively shorter limbs than
Homo sapiens.
• Skulls had sloping brows and protuberant face.
• Based on size of hypoglossal foramen and the shape of the hyoid bone, these hominins may have been able to vocalize.
• Evidence that at least 70,000 years ago Neanderthals buried their dead.


The Hyoid Bone

• Associated with skull but not directly in contact with any other bone
• Lies inferior to the mandible in anterior neck
• The only bone with no direct articulation with any other bone
• Acts as a movable base for the tongue



• Denisovans are proposed to represent a group that shares a common origin with Neanderthals
• May have been widespread in Asia and Europe during the Late Pleistocene epoch
• Disappeared around 30,000 years ago
• A 40,000 year old finger of a girl and a tooth were found in Denisova cave in southern Siberia.
• Scientists at the Max Planck Institute for Evolutionary Anthropology sequenced the genome of this archaic hominin DNA
• 4-6% of present day Melanesian genomes is Denisovan
• 1% of present day SE and East Asian genome is Denisovan
• A 400,000 year old thigh bone from Spain has DNA that resembles Denisovan DNA


Homo rhodesiensis

• Lived 550,000 - 125,000 years ago in Africa.
• Originally described from the Kabwe skull from Kabwe, Zambia (which was formally part of Rhodesia) and is sometimes referred to as the Rhodesian Man.
• Brain size 1000-1100 cm3.
• Likely emerged as an African clade off of Homo erectus.
• Homo rhodesiensis appears to be the ancestor to Homo sapiens.
-Homo rhodesiensis as likely ancestor to Homo sapiens


Homo floresiensis

• In September 2003, an archeological expedition to the Liang Bua cave on the island of Flores in Indonesia, uncovered the skull of an 18,000 year old hominid and subsequently more bone specimens were found dated at 38,000 years old.
• Sophisticated stone tools were also found at the site.
• Based on the interpretation of presence of various primitive and derived skeletal features the discoverers denoted the remains to represent a species distinct from Homo sapiens, called Homo floresiensis.

• The skeleton studied was a female who was one meter tall with a brain volume of only 380 cm3.
• Late Homo erectus brain size was 980 cm3. • Homo sapiens brain size is 1350 cm3. • The researchers postulated that this new
species emerged from the consequence of dwarfing of an ancestral Homo erectus
population on Flores
• Alternative argument has presented the possibility that the specimen represents a small bodied and microcephalic modern Homo sapiens.
• Another alternative argument suggests that the remains represent a Homo sapiens affected by endemic cretinism because of low iodine levels.
• It appears to be unlikely that DNA will be able to be extracted from the putative Homo floresiensis
• Whether Homo floresiensis represents a new species continues to be debated.


Homo sapiens are upright apes

• Upright apes.
• Have particular locomotion.
• Language communication.
• Cognitive capacity.
• Capacities for intentionality & prescience (foresight).
• Capacity to develop and use technology.
• Live in social groups.
• Have uniquely developed cultural capacities.
• Culture is defined as knowledge, behavior, and
tradition within a particular community or population.


Fossil hallmarks of advanced bipedalism

• Positioning of the foramen magnum, which becomes more central as the posture becomes more upright and the skull must be supported by the spinal column.
• Changes in pelvis, hip socket, and femora


Advanced bipedalism in
Homo sapiens

• Knee allows the leg to straighten and the knee to lock which minimizes energy expenditure in supporting the body when standing upright.
• Humans have an angled femur and a change in the limb-moving muscles such as a gluteal abductors which enables legs to more easily spread apart.


Human bipedalism

• Humans have a pelvic and muscular structure which allows the center of gravity to shift only slightly on each step, unlike the chimpanzee, which waddles back and forth with a shift of center of gravity with each step.
• The ability to run first emerged in the Homo genus, and modern humans can not only sprint, but also can endure long distance running.


Costs of bipedalism

• Backpain: the upright walking position creates pressure on the intervertebral discs, vertebrae, and the sacroiliac joints.
• Prolapse of disc, with injury to the lumbar or sacral nerves innervating the lower limbs, leads to pain, paresthesia, and sometimes motor dysfunction.
• Obesity increases likelihood of backpain
• The upright posture makes injurious falls more typical in aging people with unsteady gait and declining vision.
• Osteopenia and osteoporosis increases risk of femoral neck fractures in older populations.
• In order to walk upright in a bipedal fashion, the shape of the pelvis is changed with a flattening of the pelvis and a narrowing of the pelvic canal.
• Since Homo sapiens have a significantly larger brain than its hominin ancesters, delivery of the newborns head through the narrow pelvic canal can be problematic in some women.


Larger body size

• Increased body size and brain size increased nutritional energetic needs which required individuals to expand their ranges for foraging.
• Homo sapiens increase in height and body size enabled them to be able to forage longer distances
• Exposure to potential predation may have also contributed to selection for a larger and taller body size to enhance the ability to run and climb faster to escape.


Changes in hominin/hominid face, jaw, dentition compared to modern apes

• Flatter face.
• Teeth tucked under face with less protuberant jaw.
• Incisors became less prominent, canines become
smaller, and molars became larger.
• Change to a relatively L-shaped mandible.
• Less developed masseter muscles.
• Smaller zygomatic arch.
• Less developed masseter muscles and smaller zygomatic arch result in less powerful chewing action.
• Change in jaw shape and dentition resulted in a more grinding type of mastication to chew fiber-rich food


Costs of changes in hominid face, jaw, and dentition compared to modern apes

• It has been suggested that in recent centuries human infants, toddlers, and children in economically developed countries have been fed on particularly soft fiber-poor foods which has reduced the need for grinding movements and has increased risk of dental malocclusion and reduced development of mandible and maxilla causing increased impaction of wisdom teeth
• Evolutionary pathway is cultural evolution • Evolutionary pathway is mismatch with ancestral die
• The flattening of the face and the change in posture have changed the position of the eustachian tubes making it more difficult for fluid to drain from the tubes into the back of the throat
• This has resulted in an increased risk of otitis media (infection of tympanic membrane and middle ear).
• Evolutionary pathway is evolutionary anatomical constraints
• The changes in the position of the larynx to enable vocalization are associated with increased risk of sleep apnea.
• Evolutionary pathway is evolutionary anatomical constraints.


Reduced body hair on humans

• All other primates, including the apes, are fully covered with hair except for the palms of their hands and the soles of their feet.
• Homo sapiens are hairless, or have relatively low concentration of hair on most of the body, expect for head, axilla, and pubic area.
The commonly held explanation is the hair loss enables better thermoregulation, even though all primates, ungulates, and carnivores living in tropical areas are fully haired and are able to thermoregulate well.
• A hypothesis that is more plausible is that the hairlessness evolved by sexual selection.
• Pubic hair only appears at puberty and in both sexes the full pattern of sexual hair would be an indication of sexual maturity.
• Facial hair on males could also be an indication of sexual maturity.
• In females, breast development and menses could also indicate sexual maturity.
• Evolutionary pathway is sexual selection13


Sexual selection and reduced body hair in humans

• A hypothesis that is more plausible is that the hairlessness evolved by sexual selection.
• Pubic hair only appears at puberty and in both sexes the full pattern of sexual hair would be an indication of sexual maturity.
• Facial hair on males could also be an indication of sexual maturity.
• In females, breast development and menses could also indicate sexual maturity.
• Evolutionary pathway is sexual selection


The hominid brain

• Brain size both in absolute terms and relative to body size, did not change dramatically in the earliest members of the the hominin/hominid lineage.
• Ardipithecus and the Australopithecus had absolute and relative brain sizes not greatly different from those of modern apes.
• Significant brain size expansion occurred with the emergence of the Homo genus.
• There was an exponential increase from
Homo habilis up through H. erectus and H. sapiens.
• During this time period (2.5 million years ago to 100,000 years) there was an increase from 400 cm3 to 1350 cm3.


Encephalization quotient (EQ)

• Allometric calculation of the relationship between brain mass and body mass.
• EQ = brain weight/0.12(body weight)2/3. • Chimpanzee EQ = 2.0
• Austalopithicenes EQ = 2.5 • Homo erectus EQ = 3.3
• Homo sapiens EQ = 5.8
• This progressive enhancement of brain
size suggests positive selection for increased brain size.


Human brain structure

• Non-human primate brains have relatively smaller parietal and temporal lobes and simpler frontal lobes.
• Humans has larger parietal and temporal lobes and more complex frontal lobes with large neocortex.


Genomic changes that make us human

• Molecular genetics have identified a number of genomic regions that have shown particular change since the most recent common ancestor of humans and chimpanzees 7-8 million years ago.
• These ‘human-specific’sequences contribute to some of the unique features of Homo sapiens.


Genetic regions which influence brain development




(human accelerated regions)
• HARs (human accelerated regions) are genomic regions showing accelerated rates of base substitution since the human/chimpanzee divergence.
• HAR1 region shows the most accelerated rate of change.


HAR1F & fetal neocortex

• HAR1F (F stands for forward transcript) codes for an RNA molecule that is expressed in the fetal neocortex during development, and is co-expressed with a protein responsible for specifying cortical structure.
• The specific expression of HAR1F and rapid evolution of the human lineage suggest that it plays a role in the development of the highly expanded neocortex that is a unique human characteristic.


MCPH1 (gene microcephalin)

• MCPH1 (gene microcephalin) is an important regulator of brain size and appears to promote the proliferation of neural progenitor cells.
• Defects in this gene in humans can cause severe reduction in brain size.
• The rate evolution of MCPH1 has dramatically accelerated in the evolutionary lineage leading from ancestral primates to humans.
• These observations suggest that molecular evolution of MCPH1 may have contributed to the expansion of the brain size in the human lineage.


Genetic regions which influence limb development

• Enhancer region of HACNS1 • GBX2


Enhancer region of HACNS1 and human limb development

• Enhancer region of HACNS1 (human-accelerated conserved non-coding sequence-1) has evolved rapidly since the split in human/chimpanzee lineage.
• HACNS1 appears to regulate genes involved in limb development, especially limb-joint regions and anterior parts of forelimb and hindlimb buds.
• It appears that the human-specific sequence changes in HACNS1 contribute to the unique human digit and limb patterning, including the opposable thumb and possibly also modifications in the ankle and/or foot that allow for efficient bipedalism.


Enhancer region of HACNS1, the GBX2 gene and human limb development

• HACNS1 region influences limb development and has showed an intriguing cluster of human specific nucleotide substitutions at positions that were otherwise conserved among non-human vertebrates.
• A nearby gene, GBX2, encodes a transcriptional regulatory protein that is expressed in developing limbs


Increased brain size enabled Homo species to develop the following

• Language
• Find adaptive advantage in social interactions within their group
• Enhanced tool making abilities


Adaptive advantage in social interactions associated with increase in brain size

• The increasing societal pressures require progressively greater intellectual complexity to maintain reproductive opportunity.
• Thus a series of feed-forward loops appear to be created in which rising social complexity requires greater intelligence.


Feed forward loops in cognitive development and
selective pressure for increased brain size

Rising social complexity.
Greater intelligence.
Development of more sophisticated technology.
More complex ways of living.
More sophisticated social structures.
Increased need for higher cognitive function.
Development of language permitted and expedited the increasing neural complexity.


Types of intelligence

• IQ = intellectual quotient
• EQ = emotional quotient
• SQ = social quotient


Traits unique to humans

• Large brain size.
• Craniofacial morphology.
• Complex behavioral and cultural traits.
• Vertebral, limb and digit innovations.
• Bipedal upright walking.
• Pelvic anatomy.
• Reduced hair cover.
• Many of the DNA-level changes responsible
for unique human innovation involve multiple genes.