Animals: Mammals

Question 1

Mammals

Answer 1

Synapsids

Mammals (class Mammalia)
- >6,400 extant species.
- Includes the largest extant animals (blue whale) and some of the most intelligent, such as elephants, cetaceans, and primates.
- Basic plan body of mammals: four-legged terrestrial animals, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs.
- Mammals are amniotes that have body hair and produce milk.

Question 2

Shared Derived Traits of Mammals

Name all seven

Answer 2

1. Mammals have mammary glands that produce milk to nourish their offspring
2. Mammals have hair (or fur) covering their bodies at some stage of their life cycle.
3. Mammals have three middle ear bones
4. Mammals are endothermic
5. Dentary-squamosal jaw joint
6. Differentiated (specialized) teeth
7. Larger Brains

Question 3

Mammary Glands

Answer 3

Mammals have mammary glands that produce milk to nourish their offspring.
- Mammary glands allow mammals to raise their young independent of environmental conditions.

Origin of mammary glands is unclear.
- Soft tissues are poorly fossilized.
- Hypothesized to have evolved from sweat glands.

Question 4

Mammal Hair

Answer 4

Mammals have hair (or fur) covering their bodies at some stage of their life cycle.
- Fine, cylindrical keratin fibres.
- Hair serves multiple functions including insulation, protection, and sensory perception (whiskers).

Question 5

Mammal Ears

Answer 5

Mammals have three middle ear bones.
- Formed by the relocation of ‘leftover’ jaw bones to the middle ear.
- The mammalian arrangement of ear bones allows the detection of higher frequency sounds than reptiles, which have one ear bone.
- The mammalian eardrum is located deep in the head (better protected) compared to reptiles.

Question 6

True or false

Mammals are endothermic

Answer 6

True

Mammals are endothermic.
- Mammals can regulate their body temperature independently of the external environment.
- Independent origin from avian endothermy.
- Hair and fat also help with thermoregulation.
- Endothermy, together with a four-chambered heart, supports a high metabolic rate.

Question 7

Dentary-squamosal jaw joint

Mammals

Answer 7

Mammals have a unique jaw joint formed between the dentary (lower jawbone that carries the teeth) and squamosal (small cranial bone) bones.
- Articular and quadrate reduced in size and incorporated into the ear.

In most gnathostomes, incl. early synapsids, the jaw joint consists of the articular (small bone; back of lower jaw) and the quadrate (small bone; back of upper jaw).

Question 8

Differentiated (specialized) teeth

Mammals

Answer 8

Mammalian dentition is much more complex and differentiated than reptile dentition.

Greater diversity of tooth morphology both within an individual and between species.
- Mammals have specialized teeth (incisors, canines, premolars, and molars) adapted for different functions such as cutting, piercing, tearing, and grinding.
- Dental diversity reflects the wide range of diets among mammals.

Mammalian teeth also occlude more closely than those of reptiles.
- Allows food to be ground more finely.
- Finely ground food has a greater surface area, permitting more rapid digestion.

Mammals have two sets of teeth over their lifespan.
- Milk teeth and adult teeth.
- cf. most reptiles continually replace teeth over their lifespan.

Question 9

Mammal Brains

Answer 9

Mammals typically have larger brains compared to other vertebrates of similar size, which supports complex behaviours and higher cognitive functions.

Question 10

Mammal Limb Placement

Answer 10

The vertical placement of limbs beneath the body.
- cf. lateral placement in most reptiles.
- Vertical limb placement better supports the body, enabling more efficient movement.
- Limbs modified for running, jumping, burrowing, climbing, and swimming.

Question 11

Role of the diaphragm

Mammals

Answer 11

he diaphragm contributes to lung ventilation.
- The diaphragm is a sheet of muscle at the base of the rib cage.

Question 12

When did mammals become the dominant tetrapods?

Answer 12

Permian diversification
- Synapsids evolved during the Carboniferous period and became the dominant tetrapods during the Permian period, evolving into diverse herbivorous and carnivorous forms.
- Early non-mammalian synapsids lacked hair, had a sprawling gait, and laid eggs, e.g. reptile-like ‘sailbacks’ like Dimetrodon.
- The Permian mass extinction drastically reduced synapsid diversity

Question 13

Origin of Mammals

Answer 13

By the late Triassic, some synapsids evolved mammalian traits like hair, small size, and higher metabolic rates, though they still laid eggs.
- Included numerous Therapsida and Cynodont species.

True mammals emerged during the Jurassic period and diversified into many short-lived lineages.
- All non-mammalian synapsids are extinct.

Mammals rapidly diversified after the Cretaceous mass extinction.

Question 14

What are the three lineages of mammals

Answer 14

Monotremes
- Lays eggs
- no nipples
- cloaca

Marsupials
- Births partially developed embrios
- has nipples
- Cloaca

Eutherians
- Births fully developed babies
- has nipples
- No Cloaca (seperate systems for urine, feces, and reproduction)

Question 15

Monotremes

Mammals

Answer 15

Monotremes (order Monotremata), including echidnas and the platypus, are a small group of egg- laying mammals.
- Only five extant species (1 platypus and 4 echidnas) that are found in Australia and New Guinea.
- Monotremes lay eggs (oviparous) but provide milk to young when they hatch.
- Monotremes have no nipples; milk is secreted from patches of specialized hairs.
- Monotremes have cloacas.
- Single opening for digestive, urinary, and reproductive systems.
- Monotremes feed on invertebrates and lack teeth as adults

Question 16

Shared Derived Traits of Marsupials and Eutherians

Name all three

Answer 16

Viviparous development of embryos within the parental body, giving birth to already-developed offspring.

A placenta provides initial nourishment to the embryo.
- The placenta consists of extraembryonic membranes and the lining of the uterus.
- The placenta exchanges nutrients, metabolic waste, and dissolved gasses between the mother’s and embryo’s blood.

Post-birth nursing on milk from nipples.
- Nipples are conical outlets from the mammary glands

Question 17

Marsupials

Answer 17

Marsupials (order Marsupialia) include opossums, kangaroos, and koalas.
- ~360 extant species, all but one in the southern hemisphere.
- Marsupials are the dominant native mammals in Australia because they evolved in the absence of eutherian mammals.

Embryo development within a placenta in the mother’s uterus is very brief.
- The placenta is relatively simple and less efficient in marsupials than the well-developed placenta in eutherian mammals.

Marsupials are born as partially developed embryos.
- Crawl to mother’s maternal pouch (marsupium).
- The pouch encloses milk glands and nipples in most species.
- Embryonic development is completed while nursing in the marsupium.
- Some marsupials don’t have a pouch: the young hang on to the mother’s fur.

Both male and female marsupials possess a cloaca.

Question 18

Eutherians

Mammals

Answer 18

Eutherians (clade Eutheria) are non-marsupial mammals (~6k species).

Eutherians are distinguished from marsupials by their more complex placenta (eutherian shared derived trait).
- Eutherian embryos complete embryonic development within the uterus joined to the mother by the placenta.
- The placenta provides nourishment to the embryo throughout its development.
- Eutherians are commonly called‘ placental mammals’, but that implies that marsupials don’t have a placenta.

Post-birth, eutherian young suckle milk from nipples.
- Nipples are not contained in a pouch.
- # of nipples indicative of typical the number of offspring during pregnancy.

Eutherians have separate urinary and anal openings (no cloaca).

Question 19

Diversity of eutherians

Mammals

Answer 19

Eutherians have undergone extensive adaptive radiation, resulting in diverse body forms, habitats occupied, and feeding behaviours.

Molecular phylogenetic analyses suggest that eutherians can be grouped into four major lineages, all of which diverged from common ancestors.

Question 20

Primates

Mammals

Answer 20

Order Primates (~500 extant species) include lemurs, tarsiers, monkeys, and apes.
- Humans are members of the apes

Primates evolved from mammalian ancestors that inhabited the trees of tropical forests.
- Many primate characteristics are adaptations to life in this challenging three-dimensional environment.
- Most primate species are at least partially arboreal (living in trees).
- Some primates (e.g. some great apes) are primarily terrestrial rather than arboreal, but all species possess adaptations for climbing trees.

Question 21

Shared Derived Traits of Primates

Name all four

Answer 21

1. Grasping hands and feet
2. Flexible limbs and mobile shoulder joints
3. Forward-facing eyes
4. Large brain relative to body size

Question 22

Grasping hands and feet

Primate Traits

Answer 22

Primates have highly flexible fingers and toes that allow for grasping and manipulating objects.

Opposable thumbs: the thumb (and in some species, the big toe) is positioned apart from the other digits.
- Enables a precise grip, crucial for handling tools, food, and climbing.
- More developed in humans than in other primates, enhancing fine motor skills.

Most primates have flat nails with sensitive tactile pads, allowing for better control and touch sensitivity.
- Other mammals that rely on claws for gripping or digging.

Question 23

Flexible limbs and mobile shoulder joints

Primate Traits

Answer 23

Primates evolved flexible limbs that enable both arboreal (tree-dwelling) and terrestrial (ground-dwelling) movement.
- Ball-and-socket shoulder joints enable a wide range of motion, crucial for climbing and swinging (brachiation).
- Mobile hip joints allow for varied locomotion, including climbing, walking, and leaping.

Question 24

Forward-facing eyes

Primate Traits

Answer 24

Forward-facing eyes form overlapping fields of vision to provide depth perception (binocular vision), essential for accurately judging distances.
- Enhanced focus and detail recognition.

Primates rely more on vision than smell compared to many other mammals.
- Attributable to shortening of the snout.

Most primates can distinguish colours, aiding in identifying ripe fruit, detecting predators, and social communication.

Question 25

Large brain relative to body size | Primate Traits

Answer 25

Increased brain size compared to other mammals, particularly in regions controlling **learning, memory, and problem-solving**. **Enhanced ability to learn** from experience and adjust behaviour. - Greater reliance on learning over instinct. - Tool use and cultural transmission are seen in many primate species. Primates exhibit **complex social behaviour** and provide **extended parental care**. - Primates possess social intelligence, including complex communication and cooperation. - An extended juvenile phase allows young primates to develop survival skills through observation and practice.

Question 26

Phylogeny of extant primates | Name the two major existant groups

Answer 26

**Basal extant primates**: − Lemurs, lorises, and bush babies. − Tarsiers. **Anthropoids**: monkeys and apes. - Monkeys first evolved in the Old World (Africa/Asia); New World (South America) monkeys appear ~25 mya. - New World and Old World monkeys diversified independently following their separation.

Question 27

Apes | Primates

Answer 27

Apes diverged from Old World anthropoids about 20-25 mya. - **Apes lack tails** and have **relatively large brains**. Apes include: - Gibbons (lesser apes). - The great apes: orangutans, gorillas, chimpanzees, bonobos, and humans.

Question 28

Humans | Primates

Answer 28

Anatomical, molecular, and fossil evidence indicate that humans (Homo sapiens) are descendants of the great ape lineage. - **Humans are great apes** with **large brains and bipedal locomotion**. - Great apes, including humans, form a **monophyletic** group. - While humans have unique traits, so do other great apes, making the distinction between them less clear. - Homo sapiens is a relatively young species (~200,000 years old) and is the only extant species of the genus Homo.

Question 29

What are the closest living relatives to humans?

Answer 29

**Molecular evidence** shows that **chimpanzees (Pan troglodytes) and bonobos** (Pan paniscus) are the closest living relatives to humans. - **Genetic similarity**: The chimpanzee and bonobo genomes shared ~99% of their DNA with humans. - Meaning we shared a common ancestor, not that humans evolved from chimpanzees! - **Genetic divergence**: Around 80 genes have been lost in humans compared to chimpanzees, impacting traits like jaw size and cranial capacity. - Changes in regulatory genes can have large developmental and phenotypic effects

Question 30

Hominins | Primates

Answer 30

Hominins (clade Hominini) include chimpanzees and bonobos, and humans and their extinct relatives

Question 31

Shared Derived Traits of Humans | Name all four

Answer 31

1. **Obligate bipedalism** 2. Larger brains capable of **complex language, symbolic thought, artistic expression**, and the **manufacture and use of complex tools** 3. **Reduced jaw and facial structures** 4. **Extended childhood and learning**

Question 32

# True or False Humans are the only primates fully adapted for walking upright as the primary mode of locomotion

Answer 32

**True** While other primates can walk on two legs, humans are the only primates with bone structure requiring it

Question 33

What selective pressures pushed for bipedalism | Humans

Answer 33

Early hominins faced significant challenges favouring natural selection for bipedalism: - **Large mammal predators**. - Competition from terrestrial primates. - Changes in habitats, including a **decline in arboreal environments and expansion of open grasslands and woodlands.**

Question 34

# Four advantages, two disadvantages Pros and Cons of bipedalism | Humans

Answer 34

**Advantages of bipedalism**: - **Enhanced visibility** of predators and prey. - **Energy-efficient locomotion**, especially for long-distance travel. - Better **thermoregulation** in open habitats. - Maximum breeze; minimum exposure of body surface to the midday sun. - Freed hands for carrying food and infants, and for tool use. **Disadvantages of bipedalism**: - Vulnerability to **foot injuries**. - **Increased strain** on hips, backs, knees, and ankles.

Question 35

# Name 5 Skeletal adaptations for bipedalism | Humans

Answer 35

**Foramen magnum repositioning:** Repositioned to a central position beneath the skull to support upright head balance. **S-shaped spine**: Development of an S-shaped spine to align the upper body over the pelvis, improving balance and shock absorption. **Pelvic reconfiguration**: Shortened and broadened pelvis stabilizes internal organs and supports the upper body during upright walking. **Leg lengthening**: Longer lower limbs, with larger knee joints, increase stride length and walking efficiency. **Modified foot structure**: Arched foot with an enlarged heel and non- opposable big toe enhances bipedal stability and locomotion.

Question 36

# Name and explain all four Adaptive trends in hominin evolution

Answer 36

**Early bipedalism**: The transition from arboreal to terrestrial living prompted the shift to walking on two legs, offering energy-efficient locomotion and freeing the hands for other tasks. **Tool use**: The emergence and refinement of tool-making marked a significant milestone, facilitating dietary changes, improved hunting strategies, and cultural development. **Pelvic modification**: Structural changes in the pelvis supported an upright posture and a more efficient bipedal gait, ensuring stability and effective weight distribution. **Brain expansion** (later): A gradual increase in cranial capacity led to enhanced cognitive abilities, enabling complex problem-solving, social interactions, and language development.

Question 37

Australopiths | Primates: Hominins

Answer 37

**Australopiths** were a diverse group of early hominins living between 4–1.5 mya. - Australopith phylogeny is unresolved but is considered **paraphyletic**. - Australopiths were successful, persisting for ~3 million years. - Some Australopith species, such as ***Australopithecus afarensis*** (“Lucy” 1974) were fully bipedal (~3.2 mya).

Question 38

Characteristics of Australopiths | Primates: Hominins

Answer 38

Australopiths were **fully bipedal**, with a short, bowl-shaped pelvis and upright toe alignment but retained long forearms and curved fingers for climbing. - **Short in stature**, ranging from 1.2 to 1.4 meters (~4–41⁄2 feet) tall, and exhibited significant **sexual dimorphism**, with males being up to 50% larger than females. Australopiths had **small brains**. - Australopith cranial capacities range from 375 to 550 cubic centimetres (cc), considerably smaller than the earliest Homo species (550–750 cc). - Australopiths had relatively **large, protruding jaws**. Australopiths displayed **early evidence of tool use** dating back ~2.5 million years, predating the evolution of larger brains in later Homo species

Question 39

Homo ergaster and Homo erectus | Primates: Hominins

Answer 39

***Homo ergaster*** and ***Homo erectus*** were the **first fully bipedal and large-brained hominins**. - Larger cranium and brain capacity, e.g. H. erectus: ~1,000–1,200 cc. - Long, slender legs with hip joints **adapted to long-distance walking**. - Developed more sophisticated tools. - Reduced sexual dimorphism: increased pair bonding and reduced male-male competition. ***Homo erectus*** was the **first hominin species to migrate out of Africa**. - Spread into southern Europe, China, and Indonesia. - The oldest H. erectus fossils outside Africa date back to 1.8 mya. - H. erectus likely lived in hunter-gatherer societies and demonstrated the ability to control fire, representing a significant advancement in cultural and behavioural complexity.

Question 40

*Homo heidelbergensis* | Primates: Hominins

Answer 40

Around 0.7 mya, Homo heidelbergensis likely evolved from African populations of Homo erectus. - Homo heidelbergensis was similar to Homo sapiens in body proportions, dental adaptations, and cognitive ability (cranial capacity ~1,200 cc). - Homo heidelbergensis was a large-animal hunter, producing sophisticated tools. - **Neanderthals and Homo sapiens likely evolved from distinct populations of Homo heidelbergensis.** - Neanderthals evolved from European H. heidelbergensis populations around 250 kya. - Homo sapiens evolved from African H. heidelbergensis populations ~200 kya

Question 41

Neanderthals

Answer 41

Neanderthals (Homo neanderthalensis) l**ived in Europe and the Near East** from 350–28 kya. - Neanderthals were **cold-adapted hominins** with **robust physiques and complex behaviours**, like burying their dead and making hunting tools. - Neanderthals had brains comparable in size to modern humans. Neanderthals **coexisted with Homo sapiens for at least 40 kya**, sharing Europe for up to 5,000 years. - There is evidence of interbreeding between Neanderthals and Homo sapiens. - Neanderthals have contributed 1-4% of the DNA of humans of Eurasian descent, while this is rare in most sub-Saharan African populations.

Question 42

Denisovans

Answer 42

Denisovans were a genetically distinct group of Homo that **lived in Eurasia** around 50 kya. - **Denisovans and Neanderthals shared a common ancestor** that diverged 400–700 kya. - **Denisovans interbred with both Neanderthals and modern humans.** - Analysis of Denisovan DNA from remains found in Denisova Cave, Siberia, shows Denisovan DNA is present in modern human populations, particularly in East Asia, Southeast Asia, and Oceania. - Like Neanderthals, **Denisovans eventually became extinct**. - Denisovan taxonomic status pending more complete fossil material

Question 43

Spread of modern humans

Answer 43

Humans likely originated from Africa Humans (Homo sapiens) flourished and expanded on the African continent. **The “Out-of-Africa” migration began < 200 kya**. - By ~100 kya humans spread into Eurasia, eventually expanding across the globe into Australia and the Americas. - Humans first arrived in the Americas during the last Ice Age. - **Humans replaced other hominin species encountered**, including Neanderthals in Europe and similar groups in Asia.

Question 44

Genetic Evidence for the "Out-of-Africa" migration hypothesis | Humans

Answer 44

Phylogenetic analysis of mitochondrial DNA (mtDNA) supports the hypothesis for Homo sapiens originating in Africa. - mtDNA, inherited matrilineally without recombination, supports the “Out-of-Africa” migration hypothesis. - Phylogenies using Y-chromosome data reach the same conclusion. - Humans trace back ~200,000 years to a most recent common ancestor (MRCA) in Africa. - The MRCA most likely lived in East Africa when modern humans emerged as a distinct population. - The MRCA originated later than *Homo heidelbergensis* and the emergence of Neanderthals, but earlier than the Out-of-Africa migration.

Question 45

Genetic Variation of Modern Humans

Answer 45

As a species, humans have **relatively little genetic variation** compared to other mammal lineages. - Reflects the young evolutionary age of humans (~200 kya). - On average, the DNA of any two individuals is ~99.9% identical, i.e. only about 1 in every 1,000 DNA base pairs varies among individuals. - Because humans left Africa very recently (in evolutionary terms), there has been little chance for genetic differences to accumulate between geographically separated populations. **Most variation among humans occurs within populations**. - As much as 85% of genetic variation in humans can be found within a single population. - Only about 7% of human genetic variation segregates between groups that are commonly called races. - Some racial differences, such as skin colour, resistance to malaria, and lactose tolerance have evolved through natural selection, reflecting adaptations to local environments.