Natural vs. Sexual Selection

In The Origin of Species (1859), Charles Darwin wrote that species evolve via random mutations.
Environmental changes–e.g., changing food sources, predation, climate–favor one mutation
over another. He called this process natural selection.
The conventional view is that our smarter, larger-brained ancestors invented tools, and then
dominated their smaller-brained relations. The archaeological facts don’t support this "man the
toolmaker" hypothesis.
Our ancestors first used stone tools 2.5 million years, or 100,000 generations, ago.5 This book has
about 50,000 words. To refer to the first human as your "great-great-great…grandparent,"
you’d have to replace every word in this book with "great," and you’d need two books.
After one million years, or near the end of the first book, our ancestors’ brains were more than
double in size. Archaeologists can see slight improvements in their stone tools.6
500,000 years ago–halfway through the second book–our ancestors’ brains were nearly as big as
our brains. Our ancestors started using fire.7 Fire enabled them to move from Africa to colder
Europe and Asia.
50,000 years ago–eight pages from the end of the second book–our ancestors’ brains reached
modern size. Their stone tools became thinner and sharper. They carved small ornamental
figurines from ivory, shell, and stone. They created beautiful cave paintings. They built the first
ocean-going boats.8
5,000-10,000 years ago–the last page of the second book–our ancestors developed agriculture.
Poor nutrition made farmers’ bodies and brains smaller. They invented writing and metal tools.
They invented the bow and arrow-a weapon that seems primitive to us.9
Our ancestors’ brains enlarged before technological advances. Our ancestors’ brains were required
for each new technological innovation. Tool usewas one ofmany uses for our large brains.
Something else drove human brain evolution.
Sexual Selection
In The Descent of Man (1871), Darwin wrote that natural selection failed to explain human evolution.
Instead, he proposed an alternative theory. Species evolve whenmales and females select
each other for certain qualities. He called this sexual selection. Biologists ignored this idea for
over a century.Female mammals, in general, are more selective than males. Females in most mammal species
do most of the work of producing and raising children. In contrast, fathering offspring is less
work, so males aren’t so choosy.Females choose males with features that make the males less able to survive.11 E.g., a peacock’s
bright colorsmake him visible to predators, and his huge tail slows his escapes. His beautiful tail
communicates to peahens that he’s an especially fit individual, i.e., he’s so fast that he can escape
predators despite his heavy tail. Sexual selection is, in general, the opposite of natural selection.
Natural selection advances via slow environmental change. Natural selection advances evolution
only in harsh environments (e.g., predation, climate change). Natural selection produces
animals better able to survive–usually smaller, more efficient, and less conspicuous.
In contrast, sexual selection advances with each generation. Sexual selection produces rapid
evolutionary changes. Sexual selection advances evolution in stable environments. Sexual selection
produces animals (especially males) less able to survive, with bigger, brighter, or exaggerated
features.