sentences:  Responding to similar criticisms from viewers, Douglas Adams wrote, "If the programme didn't move and take a few risks then it would have died of boredom years ago".Several viewers wrote to point out the discrepancy between the start of life on Earth of 4,000 million years ago and the date given in "City of Death" of 400 million years ago.Graham Williams replied, "The good Doctor makes the odd mistake or two but I think an error of 3,600 million years is pushing it!