Evolution Chapter 11

• Mammals: warm-blooded vertebrates belonging to the taxonomic class Mammalia that have mammary glands, hair or fur, three middle ear bones, and one lower jawbone 
• Features of mammals: 

  • A variety of teeth to either tear food apart or grind food down
  • One single lower jawbone attached directly to the skull (powerful bite)
  • Three bones in their middle ear to aid hearing
  • Various amounts of hair or fur on their bodies throughout their lifetime
  • Mammary grands that produce milk to feed to their offsprings
• Primates: the highest order of mammals, comprised of about 400 different living species who share a number of features including opposable digits and binocular vision
• Features of primates

  • 3D colour vision and forward-facing binocular eyes - allow primates to see in 3D and locate objects in their vision
  • Large number of sensitive touch receptors in their fingertips help primates use their hands to gather information
  • Large cranium relative to body weight
  • Flexible spines and rotational hips and shoulders - increasing flexibilities and allows primates to live in trees
  • Prehensible hands and feet with five digits each, with an opposable digit (thumb or big toe) to help grasp objects with power and precision
• Hominoids: members of the superfamily Hominoidea that includes apes and humans
• Features of hominoids

  • Increased cranium size as hominoid brains tend to be larger and more complex than other primates
  • Shorter spine between rib cage and pelvis - help sit up right
  • Molar teeth in the lower jaw, which have five cusps arranged in Y5 pattern
  • Broader rib cage and pelvis - help them sit upright by creating a wide, stable chest
  • Lack of tail - to sit upright
  • Typically longer arms than legs (except homo sapiens), shoulder blades that sit back, shoulder joints that allow arms to swing behind the head
• Hominins: members of the taxonomic tribe Hominini that includes modern humans and our upright-walking ancestors
• Features of hominins

  • The ability to walk erect on our hind legs - bipedalism: using two legs to walk upright
  • structural consequences of bipedalism – centralised foramen magnum, S-shaped spine, broader rib cage, bowl-shaped pelvis, increased carrying angle of the femur 
  • communication and formation of complex social groups
• Hominin evolution

  • our ancestors gradually moved from an arboreal lifestyle to one of complete bipedalism, and that this was facilitated by the shortening of our arms and the lengthening of our legs. 
  • our faces gradually became flatter, and our skulls rounder and smaller
• • ​​Hominin Brain Evolution: Over time, hominin brains increased in size, leading to higher cognitive processes (e.g., planning, speech, abstract thinking).
• • Brain Size as a Defining Feature: Larger brains relative to body size is a key hominin trait, showing a gradual increase in size across species.
• • Fossil Evidence: Brain size is inferred from fossilised crania, with a threefold increase in brain volume from early australopithecines to modern humans.
• • Evolutionary Rate Debate: Disagreement exists on whether brain size evolution was gradual or occurred in rapid spurts due to environmental stressors.
• • Dietary Influence: Improved diets, including more fruit and animal products, played a significant role in brain size increase; cooking and fire use around 800,000 years ago further fueled brain growth.
• • Brain Complexity: Increased brain size led to more cerebrum folding, higher neuron counts, and greater brain cell connections, enhancing cognitive abilities like speech and complex emotions.
• Changes to skull shape over time:

  • a more centralised foramen magnum (1)
  • a shrinking of the sagittal crest (2)
  • a lessening of the brow ridge (3)
  • a flattening of the face (4)
  • a less protruding chin (5)
  • a more domed skull (6)
  • smaller teeth (7)

  
• Limb Structure Evolution: Hominin limb structure changed over time, with a decreased arm-to-leg ratio due to increased reliance on bipedalism.
• Shorter Arms: As hominins transitioned from tree-dwelling to ground-based living, shorter arms became advantageous, freeing them for tasks like carrying, tool use, and food preparation.
• Longer Legs: Longer legs were favoured in bipeds, improving stride length and energy efficiency in upright walking; foot adaptations like shorter toes, aligned big toes, dual arches, and wide heels further enhanced bipedalism.
• Pelvis Shape Change: Hominin pelvis shape became shorter and more bowl-shaped, supporting the upper body during upright walking; later species developed angled leg attachment to the pelvis, improving walking efficiency.
• Childbirth and Pelvis Adaptation: As hominin brain size increased, pelvis shape evolved to accommodate larger-brained offspring, leading to a narrower pelvis with a more circular birth canal, reflecting overall changes in body shape.
• The incompleteness of the fossil record makes it difficult to interpret. A few reasons for this:

  • Fossilisation is not always successful as dead organisms may not be in the right conditions
  •  Rock layers and the fossils they contain might erode and disappear
  • Many rock layers are still inaccessible
• Did we interbreed with Neaderthals?

  • Neanderthals were modern humans’ close evolutionary cousins that existed in Europe and Asia ~ 40 000 and 400 000 years ago
  • mtDNA from Neanderthal fossils compared to Homo sapiens suggests that we are separate species that shared a recent common ancestor
  • Neanderthals share similar features with humans but differ in important ways: wider nose, shorter limbs, stockier build, flared ribcage, sloping forehead, enlarged brow ridge, larger cranial capacity, resistance to cold weather
  • evidence suggests that crossbreeding occurred.
• • 1-4% of nuclear DNA between humans and Neanderthals is identical - only found in genomes of non-African populations and not in sub-Saharan African genomes
  • Neanderthals may have interbred with humans as they left Africa in the Middle East ~ 65 000 years ago and did not breed with African humans
• • 100 000 year old DNA from Neanderthal fossils found in Siberia contained human DNA not found in other Neanderthal populations
  • A population of neanderthals in Siberia may have interbred with an early form of humans - suggests a second interbreeding event with humans
• New Hominin species

  Homo Denisova

  • In 2008, a bone fragment was discovered in Siberia. 
  • Researchers at first thought is would be Neanderthal in origin. But once its DNA was sequenced it was determined to be a new hominin 
  • Denisovans seemed to have arisen in the middle east and some interbreeding occurred with Neanderthals before they headed into Asia.
• Homo luzonensis

  • Relatively small-bodied hominin ancestor that lived in the Philippine island of Luzon ~ 50 000 to 67 000 years ago
  • Show a mix of both ancient and modern hominin traits, which made it difficult to identify the bones as belonging to an existing species
  • Specimens shows a new combination of features that were different from the combination of features found in other species in the genus Homo, hence are believed to warrant their classification as belonging to a new species
• Multiregional hypothesis a model for the geographical spread of Homo sapiens which suggests that separate human populations evolved independently from earlier hominins that had spread throughout Eurasia and experienced gene flow. Also known as the regional continuity model

  • No longer a widely accepted theory
  • Homo sapiens evolved in a specific location in eastern Africa
  • Hominin populations around the world maintained gene flow
  • Homo sapiens evolved simultaneously in multiple locations around the world
• Out of Africa hypothesis a model for the geographical spread of Homo sapiens which suggests that humans first developed and evolved in Africa before migrating outwards and expanding their colonies, replacing the earlier hominins that had spread prior. Also known as the African replacement model.

  • The more widely accepted theory
  • Hominin populations around the world were genetically isolated
  • Homo sapiens migrated out of Africa about 100,000 years ago and displaced and interbred with other hominins
  • Supported by mtDNA evidence
• Both hypotheses agree that the Homo lineage originated in Africa and expanded to Eurasia about 1.8 million years ago with the departure of Homo erectus – but they differ in explaining the origin of modern humans.
• mtDNA:

  • mtDNA stands for mitochondrial DNA. Every mitochondria has its own circular chromosome separate from the DNA in the nucleus.
  • mtDNA is inherited only through the maternal line as only the mitochondria from the egg cell become part of the zygote.
• Out of Africa hypothesis and mtDNA

  • Mitochondrial DNA is used to compare closely related organisms as it does not recombine and it has a higher mutation rate than nuclear DNA.
  • mtDNA provides strong evidence for the out of Africa hypothesis
  • The greatest diversity of mtDNA is found in Africa.
  • mtDNA found in all other regions of the world show similarity to just one particular lineage. 
  • This provides evidence of the founder effect. The small groups of Homo sapiens that left Africa took only a limited set of genetic information with them.
• Modern Aboriginal Australians can trace their lineage back to this first migration out of Africa, where it is believed that around 50 000 to 65 000 years ago the first humans arrived on the land. This makes Aboriginal Australians one of the oldest continuous populations on earth, and has tremendous impacts on Aboriginal peoples Connection to Country.

  Connection to Country refers to a reciprocal relationship between First Nations people and their ancestral lands and seas: the land provides for the people, while the people manage and sustain the land through their culture, ceremonies, and care.