"He was obviously in severe, excruciating pain," Dr. Nizam Peerwani, the Tarrant County medical examiner, testified before the state rested its case on the third day of the trial.
The defense also rested Wednesday after calling just one witness: a medical examiner from another county who said Biggs probably lost consciousness after his head hit the windshield.
Chante Mallard, a 27-year-old former nurse's aide, is accused of hitting Biggs as he walked along a highway, then driving home with his body lodged in her windshield, shutting the garage door and leaving him to die. Police say she sat in the car, crying and apologizing to the moaning man.
She faces life in prison if convicted.
Mallard's attorneys don't dispute that she hit Biggs early on the morning of Oct. 26, 2001, or that she had been using drugs and drinking, but they argue that Biggs' death was an accident, not murder.
The impact broke Biggs' thigh bone, shin bones and arm on his right side, nearly amputated his left leg, and gouged his torso, Peerwani testified Wednesday.
His injuries were aggravated as Mallard continued to drive, but Biggs still could have been saved with quick medical treatment, the examiner said. Instead, he said, Biggs bled to death over about two hours.
A Fort Worth Fire Department captain and an emergency room doctor had also testified that Biggs could have survived if he had been quickly treated.
The injuries the 37-year-old homeless man suffered wouldn't have prevented him from moving his hands and talking, Peerwani said.
The sole defense witness agreed that Biggs lived one or two hours after being hit, but said he probably was unconscious during the entire ordeal.
"It's doubtful that he ever regained consciousness," said Vincent Di Maio, chief medical examiner for Bexar County, who reviewed Peerwani's autopsy report.
The victim's son told jurors Wednesday that Biggs was a self-employed bricklayer who took medication for bipolar disorder and mild schizophrenia.
"I would say he was very hardworking. He was very friendly, although he didn't have many friends," said Brandon Biggs, 20. "He was very, very loving, I would say."
Mallard appeared to cry during the testimony of the son, who lived with his mother after his parents divorced but said he kept in touch with Gregory Biggs until the summer before he died.
Brandon Biggs said he sometimes took his father to movies and the mall after meeting him at a Fort Worth homeless shelter.
Biggs' body was found in a park a day after he died. Two of Mallard's friends, Clete Deneal Jackson and Herbert Tyrone Cleveland, pleaded guilty to dumping the body.
The case has stirred a Ft. Worth legal debate over whether murder charges fit the crime.
"Does bringing the guy home in your windshield and not giving him any aid and letting him die, is that murder?" local attorney Greg Westfall asked CBS News.