Preserved remains or traces of ancient organisms that provide evidence of evolutionary history.

Specialized nerve cells that receive stimuli and transmit messages.

Tip of the axon containing neurotransmitters to transfer chemical messages.

Fatty layer covering axons; insulates neurons, speeds impulse transmission, and protects axons.

Spontaneous, involuntary response to stimuli (e.g., withdrawing hand from heat).

Branched fibers of neurons that receive messages from adjacent neurons.

The longest fiber from a neuron’s cell body; transmits impulses away from the cell body.

Volume of the braincase, measured in cubic centimeters (cm³), indicating brain size.

The rearrangement of genetic material (especially during sexual reproduction), producing new trait combinations.

The hypothetical earliest organism from which all current life on Earth descends.