Begins in northern Norway, 500 kilometres north of the Arctic Circle. Here, there is only just enough light for the pine trees to survive, but it is extremely cold during the winter. Pine cone seeds provide one of the few foods available at this time of year, and large herbivores such as the moose must also rely on their fat reserves.
From the roughest seas to the crystal clear waters of the Florida springs, David Attenborough swims with sea otters and dives with manatees, as he follows those mammals who, millions of years ago, left dry land and returned to the water to feed.
There is no greater test for life than winter, as temperatures plummet to 70 below and winds reach 200kph. Darkness and ice extend across the polar regions and only a few remarkable survivors gamble on remaining. We join a female polar bear trekking into the Arctic mountains to give birth as the first blizzards arrive. Out on the frozen ocean, the entire world's population of spectacled eider ducks brave the winter in a giant ice hole kept open by ferocious currents. Arctic forests transform into a wonderland of frost and snow - the scene of a desperate and bloody battle between wolf and bison, but also where a remarkable alliance between raven and wolverine is made. Beneath the snow lies a magical world of winter survivors. Here tiny voles dodge the clutches of the great grey owl, but cannot escape the ultimate under-show predator - the least weasel. Midwinter and a male polar bear wanders alone across the dark, empty icescape. Below the snow, polar bear cubs begin life in an icy den while fantastical auroras light the night skies above. In Antarctica, we join male emperor penguins in their darkest hour, battling to protect precious eggs from fierce polar storms. Weddell seals escape to a hidden world of jewel-coloured corals and alien-looking creatures but frozen devastation follows as sinister ice stalactites reach down with deadly effect.
Our body is a true time machine. It is a mirror of the history of the living world. If an engineer today had to fabricate the ideal human, he would most likely not design us the way we are currently made. He would go to the most logical, the most efficient, the most rational; and when you look closely, this is not always what characterizes us.