Evolution and the Brain
Dr.C.V.NARASIMHA MURTHY
DEPARTMENT OF ZOOLOGY
Key points
Origin of Brain Cells and Brains
Despite the age of the Earth (4 500 million years), brain cells and brains are quite recent adaptations
– First forms of life: 3 500 million years ago
– First brain cells: 700 million years ago
– First brain: 250 million years ago
– First human-like brain: 3 - 4 million years ago
– Modern brain: 0.1-0.2 million years ago
Chordates: Invertebrate vs.vertebrate
Complexity of movement
• Nerve net:
– Sensory and motor neurons
Anemone
– Divided into a number of parts
– Bilaterally symmetrical
– Collection of nerve cells that function somewhat like a brain
Ascidian
C. elegans
Invertebrate vs. Vertebrate Nervous System
Invertebrate
Vertebrate
– Brain and spinal cord encased in cartilage/bone
Earthworm – invertebrate
– Crossed organization: Each hemisphere receives information
from and controls the opposite side of the body
– Spinal cord is dorsal at the back of the heart and gut
Evolution of Vertebrate Nervous System
• Behavioral complexity is correlated with the evolution of cerebral hemispheres and cerebellum:
– Cerebellum: involved in the coordination of motor and possibly other mental processes
– Increased size and folding (to fit more tissue into the skull)
Less to more complex vertebrates
Less to more complex vertebrates�Cortical specilasation�
Sensory/motor cortex
Frontal cortex
Less to more complex vertebrates
sensory cortex and locomotion
Less to more complex vertebrates �(i.e. rat to human)
variety of non-neuronal cells
Chordate – mammals – primates
Features common to primates:
• Excellent color vision
• Eyes in front of face: enhance depth perception
• Females: Usually only one infant per pregnancy;
infants require more care
• Larger brains
Primates hominidae
• Hominid:
– Primates that walk upright; includes all forms of humans, living and extinct
• Australopithecus
– Austral: “southern”; pithekos: “ape”
– Our distant ancestor
• Gave rise to the genus Homo, or human
– 4 million years ago
Brains were 1/3 the size of ours
• Homo habilis (“handy human”)
– 2 million years ago; in Africa
– Made simple stone tools
• Homo erectus (“upright human”)
– Migrated to Europe and Asia
– 1.6 million years ago
– Larger brain than H. habilis
– More sophisticated tools than H. habilis
• Homo sapiens (“wise human”)
– Africa and Asia: 200,000 years ago
– Europe: 100,000 years ago
– Larger brain that H. erectus
– Coexisted with other hominids
• Example: Neanderthals, who had comparable or even larger brains than H. sapiens
• Exact reason why we replaced Neanderthals is unknown
Brain size in mammals
Encephalization Quotient
Encephalization Quotient (EQ):
– Measure of brain size obtained from the ratio of actual brain size to the expected brain size for an animal of a particular body size
• H. sapiens have the largest EQ
Why is the Hominid Brain Enlarged ?�
1. The Primate Lifestyle
• Eating behavior of primates is more complex than other animals
– Finding fruit is more difficult than eating grass
or other vegetation on the ground
• Need good sensory, spatial, and memory skills
– Fruit eaters have larger brains
Why the Hominid Brain Enlarged
2. Changes in Hominid Physiology
Radiator Hypothesis (Falk, 1990)
– The more active the brain is, the more heat it generates
Stedman and colleagues (2004)
Why the Hominid Brain Enlarged
3. Neoteny
• Rate of maturation is slowed
– Allows more brain cells to be produced
• Adults retain some infant characteristics
• Newly evolved species resemble the young of their common ancestors
– Human heads look more like the heads of juvenile chimpanzees than adult chimpanzees
Modern Humans
Are people with larger brains more intelligent?
Problems with answering this question:
• How does one measure brain size?
– Control for skull thickness
– Volume versus weight
– Control for body weight
• Can fluctuate within an individual over time
– Effect of age, physical health, brain damage?
Modern Humans
Are people with larger brains more intelligent?
• Brain size and intelligence do not seem to
be particularly related
– Research has shown that many smart people
(e.g., Einstein) have average size brains
– Women’s brains weigh about 10% less than
men, but the two sexes do not differ in measures of average intelligence
Can you name few differences between the invertebrate and vertebrate nervous system?
Invertebrate
- Nerve nets, ganglia
- Stimulus/response, receptor/effector
- Reflexes, conditioned responses
Vertebrate
– Brain and spinal cord encased in cartilage/bone
– Crossed organization: Each hemisphere receives information
from and controls the opposite side of the body
– Spinal cord is dorsal at the back of the heart and gut
What are the changes that occurred between less to more complex vertebrates?
sensory cortex and locomotion
Thank you...
The Organization of the Vertebrate Brain 3
Brain size is log-linearly related to body size. Animals above the line may be “smarter”; but does that make them more fit?
Size is not everything. Brain differentiation may be a strong indicator of neural “processing power’.
Striedter
The Vertebrate Brain Plan 1
The CNS develops from a neural tube. The fluid filled core remains in the adult as the ventricular system.
The major subdivisions of the CNS are found in all vertebrates: Forebrain (telencephalon + diencephalon, midbrain (mesencephalon) and hindbrain (rhombencephalon).
Different parts of the CNS hypertrophy or atrophy depending on the species.
A second division is useful in the mid and hindbrain: the alar plate and basal plates. The alar plate forms the roof and is generally sensory while the basal plate is motor.
Useful terms for localization are: dorsal vs ventral and rostral vs caudal.
Striedter
Butler and Hodos
The Vertebrate Brain Plan 2
The brain of a fish. The telencephalon is relatively small compared to the tectum, a midbrain visual center.
In mammals, the tectum is called the Superior Colliculus.
Sensory input is stereotyped: in most vertebrates, sensory input reaches the pallium (dorsal telencephalon) via relays in the thalamus. Olfactory input enters the pallium directly from the olfactory bulb. Motor output is usually from mid and hind brain, except in mammals where the pallium (cortex) becomes dominant. The cortex assumes ever greater importance in more “advanced” vertebrates.
The cerebellum also evolves to regulate both brainstem and forebrain neural processing.
Butler and Hodos
The Vertebrate Forebrain 1
A
B
C
Striedter
The Vertebrate Forebrain 2
Cortex increases its surface area without a large increase in volume by folding: the folds are called gyri.
The thalamus is the gateway to cortex and so evolves together with cortex. As cortex evolves areas that are not directly sensory or motor, thalamus evolves regions that connect cortex to cortex (and not to sensory input). One good example is the primate pulvinar.
Striedter
The Vertebrate Forebrain 3: Sensory
Visual areas increase enormously in the primate. And secondary visual areas increase the most. These are involved in extracting very complex information from images. In other words, the retina of a rabbit is just as complex as that of a primate; the primate extracts a lot more information from that retina and this is reflected in the increase in primary and secondary visual cortices.
Striedter
The Vertebrate Forebrain 4: Motor
A
B
C
Striedter