The beginning of Atalanta's life is both sad and confused. It is sad because she was exposed (left out to die) and confused because no one knows who her father was. We do know however that her exposure was due to his (her father's) desire for a son, and his disappointment at a daughter. She was abandoned somewhere in Arcadia. This could have been the early end for our young heroine, but the Fates would have none of that. A motherly she-bear happened to lumber by and found the baby Atalanta. Now, I wouldn't go testing this with MY kids, but the mother bear decided to suckle the babe. And so Atalanta survived just fine under the she-bear's care until some hunters came along. The hunters brought her up.
It's funny what the wilderness seems to do for beauty, because Atalanta grew to be one of the world's most beautiful women. And despite this, decided to stay a virgin. In addition to her beauty, she could out-race and wrestle any man and was an excellent huntress (like her patron goddess Artemis). For example she beat Peleus (the to-be father of Achilles) in a wrestling match, and as for racing, well, we'll get to that. She was well able to defend herself (not much of a shocker there) but before she was too well-known (or maybe it's just the stupidity of centaurs) two centaurs (Rhoecus and Hylaeus) tried to rape her. She calmly shot them down. She was enough of a warrior, and well-respected enough, that she even asked Jason if she could accompany him on the Argo. My sources say that he reluctantly declined her offer, having some idea of what turmoil might be caused with a woman on board such a long journey.
This story must branch off for a moment to tell the story of the Calydonian Boar Hunt.
There is a custom in Calydon, and then in most countries, that the first fruits of the harvest are sacrificed to the gods. This is too keep them appeased and to show respect. Unfortunately, King Oeneus 2 didn't have good help to keep his gods straight and "accidentally" omitted a sacrifice to Artemis. Whoo doggy, that wasn't such a hot idea. Artemis, furious at Oeneus's negligence, sent down upon them the Calydonian Boar. A Boar who was ENORMOUS (a son of Echidna, if I'm not mistaken), and had all the nasty qualities of a wild boar multiplied in character as much as he was multiplied in size. The Boar ravaged Calydon, destroying crops, cattle, and people. King Oeneus 2 didn't hesitate this time, and called in the noblest men and Atalanta to fight the terror. Even his son Meleager joined the team. The skin of the boar was offered as a reward to the man who could kill it. Now, the times were sexist then (though not as much as one might imagine in some ways), and many of the noblemen were not interested in fighting alongside a woman - even one with a reputation like Atalanta's. Fortunately for Atalanta, Meleager was in love with her (despite his marriage to Cleopatra - no, not THAT Cleopatra), and convinced the other men to follow her.
The hunt was on. Atalanta was the first to see the beast, and the quickest to shoot. Amphiaraus (another able-bodied young man) followed suit with an arrow to the beast's eye. Meleager, being the young dare-devil he was, and an excellent hunter took his spear and gorged the Boar. It almost did the trick, but the boar was big enough that it kept coming, and almost killed Meleager but for Atalanta's quick action. Meleager got the skin and the credit (it was his spear that had the mortal wound after all) but was enlightened and cool and infatuated enough to give it to Atalanta. Now, the other hunters were rather insecure with their own masculinity, and didn't like the idea of Atalanta, a woman, getting the boar's skin instead of the men. They were Meleager's uncles, and declared that if he didn't want the skin that was fine, but they deserved it after him, by right of blood. Meleager, in his anger, slew his chauvanistic uncles (right on!) and returned the skin to Atalanta. His mother (Althaea) couldn't forgive him for her brother's death, and killed Meleager by throwing the log into the fire that the Fates had magically tied his life with..
At this point Atalanta had more than a reputation as a good huntress, she was one of the famed Calydonian Hunters and EVERYONE wanted to be around her. I suppose it wasn't much of a surprise that her father got back in touch, becuase he did. Atalanta returned to her father's house on the conditions that she could remain a virgin. Her father agreed, but soon suitors were pushing down the doors and her father changed his mind. Sort of. He and Atalanta decided that she would marry the man who could beat her in a footrace, and any man who lost would die.
"I am not to be won till I be conquered first in speed. Wife and couch shall be given as prize unto the swift, but death shall be the reward of those who lag behind." [Atalanta to the suitors. Ovid, Metamorphoses 10.570]
They laughed together because they knew that no man would ever beat her, and that the many executions would slow the flow of suitors. Of course, some still came. Most who came still were the stupid ones, but one man, Hippomenes (or Melanion) was exceptionally smart. He went first to Aphrodite and told her of the virgin huntress and begged her help. Aphrodite, offended at the idea of virgins in general, gladly gave him three irresistable Golden Apples that he must have stared at until the race began. Thanking her he returned to woo Atalanta.
Atalanta just sighed that such a good-looking man would have to die, but wasn't THAT interested in marriage. Besides, she like winning too much to throw the race. Hippomenes threw one last prayer to Aphrodite and walked to the starting line. The race began. Hippomenes got out to a good start, he was an excellent runner himself, but soon Atalanta sped ahead of him. Quickly he threw out the first of his Golden apples slightly ahead of Atalanta and slightly off the track. Atalanta saw it and desired it and stopped to pick it up. Hippomenes passed her with delight, praying to Aphrodite while he ran. But Atalanta soon caught him and two-thirds of the way through, she passed him again. This time Hippomenes through his Golden Apple deeper into the bushes and when Atalanta stopped, it took her a moment longer. Hippomenes quickly ran by, sweating with effort and relief. But before he knew it the speedy Atalanta over took him once more, and it was just before the finish line! Hippomenes threw his last apple deep into the underbrush and prayed like mad to Aphrodite. Atalanta ran a moment more, then dashed back to the apple, she had to have it. As she emerged from the bushes Hippomenes crossed the finish line.
But now that Hippomenes had won his prize he was so happy that he forgot to THANK Aphrodite. He went instead to the temple of Zeus to celebrate his victory with Atalanta. Aphrodite was furious and sent flaming desire coursing through Hippomene's and Atalanta's veins and they lay together right there in Zeus' holy temple. Zeus was horrified at the profanation and turned the pair into lions.
Another version says the transfiguration came later in their marriage when the two were hunting together, entered Zeus' territory and slept together in his temple THEN. However it happened the two were changed into lions, and hunted together the rest of their days.
To read more about this, check out Apollodorous' account of the Calydonian Boar Hunt in his Library and Epitome, (1.8.2) and in Ovid's Metamorphoses (8.260). You can also read about Atalanta and Hippomenes in the Metamorphoses (10.560).