When working together at Zip2, Elon Musk had to go to the ER for stitches and a tetanus shot after his brother, Kimbal, once tore off a hunk of flesh from his hand

• Kimbal Musk bit Elon Musk and "tore off a hunk of flesh" off his hand during a fight in the 1990s.
• Elon Musk had to go to the emergency room for stitches and a tetanus jab.
• The incident is detailed in Walter Isaacson's new biography.

Kimbal Musk once "tore off a hunk of flesh" from Elon Musk's hand while the brothers wrestled on the floor in Zip2's office, Walter Isaacson wrote in "Elon Musk," his new biography of the tech mogul.

Kimbal Musk bit his brother because he thought Elon Musk was about to punch him in the face, the book said. Elon Musk had to go to the emergency room to get stitches and a tetanus jab as a result, the book says.

The brothers often had "rolling-on-the-office-floor fights" while they worked together at Zip2, their company that provided city guides to newspapers, Isaacson wrote. Because Zip2 didn't have private offices, other staff had to watch the two fight, per the book.

"When we had intense stress, we just didn't notice anyone else," Kimbal Musk told Isaacson.

The brothers spent a lot of time in close proximity. When they first started working on what would become Zip2 in 1995, they both slept in the office in Palo Alto for around six months before renting an apartment together.

Elon Musk has frequently been criticized for his high-intensity leadership, including how he gives feedback and his "hardcore" work culture. The Tesla CEO has often offered jobs to his close friends and family, including one college friend who only stuck it out for six weeks at Zip2. "I knew I could either be working with him or be his friend, but not both," Navaid Farooq told Isaacson.

The book describes Elon Musk's and Kimbal Musk's volatile relationship.

"I love, love, love my brother very much, but working with him was hard," Kimbal Musk told Isaacson. Topics they disagreed on included the name of Zip2, which was proposed by Kimbal Musk and a marketing firm and which his brother never liked, per the book.

The boys' parents divorced in 1979, after which the brothers and their sister Tosca lived with their mother, Maye Musk, near Durban. But around two years later, when Elon Musk was 10, he decided to move in with his father, Errol Musk, in Pretoria.

Kimbal Musk, just over a year younger than Elon Musk, also moved in with Errol around four years later. "I didn't want to leave my brother alone with him," Kimbal Musk told Isaacson.

Though they spent a lot of time with each other – including selling homemade Easter eggs door-to-door and playing "Dungeons and Dragons" with their cousins — they fell out over small things, and sometimes this turned to violence.

On one occasion, the brothers argued at a country fair. "They were wrestling and punching each other in the dusk," their cousin Peter Rive told Isaacson. "People were freaking out."

"Growing up in South Africa, fighting was normal," Elon Musk told Isaacson. "It was part of the culture."

The book describes how Elon Musk was taken to a wilderness survival camp, which he dubbed a "paramilitary 'Lord of the Flies'" when he was 12, where children were encouraged to fight each other for rations of food and water. He was beaten up twice and lost 10 pounds, Isaacson wrote.

When Elon Musk returned to the camp just before turning 16, he'd learned that if people bullied him, "I could punch them very hard in the nose, and then they wouldn't bully me again."

The book also shares more details on an infamous incident in which a young Elon Musk was pushed down the stairs at school. The book says that after a student accidentally bumped into him during assembly, Elon Musk pushed him back. At recess, the other student and his friends kicked him in the head and pushed him down a set of concrete steps.

"They sat on him and just kept beating the shit out of him and kicking him in the head," Kimbal Musk, who had been with his brother at the time, told Isaacson. "When they got finished, I couldn't even recognize his face. It was such a swollen ball of flesh that you could barely see his eyes."