Trying to find a manga with no luck:
- Action takes place in modern-day Japan, in Tokyo I believe
- The protagonist is a lazy-yet-talented heir to a martial arts dojo, with a resentful older brother
- A meteorite hits without warning, transforming a vast area into wilderness
- Those surviving find themselves able to morph into beast-like forms
- Protagonist gets Wolverine-ish form, his brother a mantis; some girl who ends up eating her brother gets a giant unconscious-only form
- A goddess explains something about "survival of the fittest / adapting to new environments" being the purpose