travel to the tropics today and you 'll experience balmy annual mean temperatures of 30 °c -lrb- -40 °f -rrb- .
it suggests temperatures at earth 's equator were -40 °c -lrb- -40 °f -rrb- 2.4 billion years ago -lrb- artist 's illustration shown -rrb- .
the research , led by dr daniel herwartz from the university of cologne , was published in the proceedings of the national academy of sciences .