It indicates that our ancestors had a different build than humans of today.
The answer is Adrenaline
Adrenaline is a stress hormone secreted by the adrenal glands and some other neurons when a person is frightened, excited or generally in stress.
It prepares our body for it's 'fight or flight' mechanism and tightens muscles for exertion.
After a bank robbery, most people feel a rush of adrenaline.
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Hormones
The endocrine system includes various glands in the body that are responsible for secreting hormones into the bloodstream. Hormones are chemical substances produced by the body that regulate certain functions of cells and organs. Hormones can regulate sleep, sex, growth, stress, huger, metabolism, etc.