Birmingham police spokesman Sean Edwards said the boy beat and killed 1-year-old Kelci Lewis when she wouldn't stop crying.
The mother of the toddler, 26-year-old Katerra Lewis, also is charged with manslaughter for her role in the death of her daughter.
"This is by far one of the saddest cases that I have witnessed and been a part of since I became a police officer," Edwards said. "This type of irresponsibility on behalf of a parent is totally unacceptable," he said.
Edwards said the 8-year-old was the oldest of six children left alone as their mothers, Katerra Lewis and a friend, went to a nightclub on the night of October 11.
More From This Section
The 8-year old is the son of Katerra Lewis' friend. The friend has not been charged.
A 6-year-old who was in the home that night told police what happened, Edwards said, and the information was consistent with the toddler's injuries. Kelci Lewis suffered severe head trauma and major damage to her internal organs, police said.
He said the mother's punishment is "something she has to live with for the rest of her life."
Katerra Lewis has posted USD 15,000 bond.