It was some kind of big cat. Eve wasn't quite sure what kind, but she knew it was alone and could sense- and now see- its intense arousal. Between its back legs she noticed a firm, pointed pink shaft, and her eyes widened as she noticed the tiny spines that covered it. It was the sight of those that convinced her to give up the purity of her primate existence- no fellow primate had such a tantalizing penis, and combined with the firm, muscular hunter's body of this feline, she knew she wanted this male, no other.
Eve lay back on the grass and presented herself like a primate, and in response the big cat settled atop her, pressing its paws to her hands to hold her in place and prepared to rut her like a feline. When its spiked cock rubbed along her pussy lips, it made her body respond in kind with a feline- she instantly started ovulating, her form preparing to give him a litter.
Just before it pushed inside her, the big cat looked down at Eve with a glint in its eye, and she leaned up to kiss it, the creature responding in kind as best as it could. And a moment later, Eve felt its shaft fill her, scraping pleasurably at her pussy walls and irrevocably blurring the line between primate and feline. Despite its unnaturalness, their mating could not have felt more natural and right.
But it was not just Eve giving in to a more animal side- the big cat started making more human gestures as the two mated. Its kissing grew more in kind with Eve's, and when Eve's hands let go of its front paws, it moved them to her breasts and brushed along her nipples firmly, making her moan. As Eve's pussy clenched tightly around its shaft, putting pressure on its spines, it grew closer and closer to climaxing.
Finally, the creature did peak, and yowled in pleasure as it shot its seed deep inside Eve. Eve moaned eagerly in response as being filled pushed her to her own peak, and as her body was fertilized, the human race ended and this new feline race's existence began.