State the trend in relative brain mass in primates other than humans according to their time of divergence from humans.

Suggest one reason that the relative brain mass of Homo is different from Parapithecus and Australopithecus.

Deduce, with a reason, whether the relative brain mass is a good indicator of brain development.

Primates belong to the phylum chordata. The neural tube of chordates is formed by the infolding of the ectoderm followed by the elongation of the tube. Outline the process of formation of neurons from this neural tube in primates.

the longer ago they diverged from humans, the greater the relative brain mass ✔

Accept inverse.

Do not accept negative correlation.

a. human cerebral cortex has extensive folding producing a greater mass ✔

b. better diet/more protein allowed increase in «relative» brain mass in humans ✔

c. others had larger body to protect them from predators «without increase in brain mass» ✔

a. not a good indicator because it depends on body mass ✔

b. not a good indicator as less developed organisms show a larger relative brain mass ✔

Accept answers referring to specific organisms shown

a. neurons formed by a process called neurulation ✔

b. neurons are «initially» produced by differentiation «in the neural tube» ✔

c. immature neurons migrate to a final location ✔

d. an axon grows from each immature neuron in response to chemical stimuli ✔

e. some axons extend beyond the neural tube to reach other parts of the body ✔

f. a developing neuron forms multiple synapses ✔

This question concerned how relative brain mass varied in primate evolution. Overall it was not well answered. The decreasing scale on the X-axis caused confusion for a lot of candidates in part (a). 

In parts (b) and (c) few candidates referred to ‘relative’ brain mass thereby providing incorrect responses.

In parts (b) and (c) few candidates referred to ‘relative’ brain mass thereby providing incorrect responses.

In Part (d) many candidates were familiar with the process of neuron formation in primates though some may have been confused by the introduction to the question.