"The Unbroken Thread" is the fourth video in the Symphony of Science series, and it features David Attenborough, Jane Goodall, and Carl Sagan. The clips used in this installment come from Carl Sagan's Cosmos, David Attenborough's Charles Darwin and the Tree of Life, The Life of Mammals, The Living Planet, BBC Life, XVIVO Scientific Animations, IMAX Cosmic Voyage, Jane Goodall's TED Talk, and a clever Guiness Commercial. The themes present in The Unbroken Thread attempt to explore the wild diversity of life on our planet, the intricacy and origin of its mechanisms, and its close relation to all other life forms. See www.symphonyofscience.com for more science music.

Lyrics:

[David Attenborough]
All life is related
And it enables us to construct with confidence
The complex tree that represents the history of life

Our planet, the Earth, is as far as we know
Unique in the universe; it contains life
Here plants and animals proliferate in such numbers
That we still have not even named all the different species

Darwin's great insight revolutionized the way in which we see the world
We now understand why there are so many different species

[Carl Sagan]
Every cell is a triumph of natural selection
And we're made of trillions of cells (Within us is a little universe)
Those are some of the things that molecules do
Given four billions years of evolution (We are, each of us, a multitude)

Now how did the molecules of life arise?

[Attenborough]
It began in the sea
Some 3 thousand million years ago
Complex chemical molecules began to clump together

These were the "seeds"
From which the tree of life developed
They were able to split, replicating themselves
As bacteria do

[Sagan]
The secrets of evolution
Are time and death
There's an unbroken thread that stretches
From those first cells to us


[Jane Goodall]
There isn't a sharp line dividing humans
from the rest of the animal kingdom
It's a very wuzzie line

It's a very wuzzie line,
and it's getting wuzzier
All the time

We find animals doing things that we,
In our arrogance,
Used to think was "just human"


[Attenborough]
Its continued survival now rests in our hands