Some of humanity’s evolutionary history can be observed in the development of a human fetus. As chicken and human embryos develop, for example, they experience a stage where they both have a tail, as well as arches and slits in their neck remarkably like the gill slits and arches found in fish. Thus, scientists in the late 20th century concluded that chickens and humans most likely shared a fish-like ancestor, based not only on visual evidence but also on DNA and fossil records. Not all ancestral characteristics become evident during fetal development, but enough similarities exist to suggest an evolutionary thread.

A few days after conception, a human embryo’s cells begin to specialize. Some form a simple neural plate, which changes into a groove and then a tube. The huge cerebral cortex that distinguishes the human brain develops last, in the final months before birth, just as it evolved from humanity’s simian ancestors two million years ago relatively late on the evolutionary tree. Like an hour-long film compressed into a few seconds, the pageant of growth and diversity in the fetal brain roughly condenses a half billion years of animal evolution into nine months of flesh and blood transformation.

The common animal ancestors of humans and other animals are suggested by common elements of animal brains. The more complex structures of the late developers overlie the simpler forms of creatures that evolved earlier, and thus lower on the evolutionary tree.


AT FIRST; Russian physiologist Ivan Pavlov (1849-1936) wanted only to know the neural link between dinner and dog drool. To find out, he anesthetized his test subject and detached its salivary duct, lightly stitching this to the dog’s outer cheek. Then, placing food in the dog’s mouth, he could eaSily collect and calculate its salivary response. In this way he hoped to unlock the mysteries of the canine nervous system.

After repeated experiments, Unfortunately the dog seemed to catch on and began to salivate before the food had arrived. Clearly this was a problem. How could Pavlov understand salivary response to food in the mouth if the response occurred in the absence of food? Initially puzzled, Pavlov realized he’d stumbled upon something even more intriguing than his original objective. As environmental factors determine evolutionary adaptations within a species, he concluded, so too must external forces mold the behavior of an individual.

Ivan Pavlov observes one of the dogs he subjected to conditioned behavior experiments.
Ivan Pavlov observes one of the dogs he subjected to conditioned behavior experiments.

From a knee-jerk defense mechanism to the performing of Rachmaninoff, acquired reflexes are the building blocks of learning. And if dogs’ brains were sophisticated enough to make such connections, imagine what human brains could do.

Pavlov soon discovered he could condition animals to respond to arbitrary stimuli. If a snack was repeatedly paired with buzzer, whistle, or A-minor triad on the piano-he rarely used that legendary bell-the dog would begin to salivate at sound alone. But a slight variation-B-flat minor, perhaps or A minor in a different octave-triggered no response. The same held for shapes, clocks, shades of gray,melodic patterns, light and rotating objects.

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