Biological Clocks – Circadian Rhythms
The cyanobacteria changed the world… turning it green. And that had a wonderful consequence. With this new way of living, life had released oxygen into the atmosphere of our planet for the first time. This article considers the biological clocks that influence the circadian rhythms of every animal and plant on the planet.
Time – The Final Ingredient
And in doing so, over hundreds of millions of years, it eventually completely transformed the face of our home. And as the oxygen levels grew the stage was set for the arrival of ever more complex creatures. But on Earth, the emergence of complex life required a rather more intangible ingredient.
Something that you can’t see, touch or smell, and yet you pass through every day… Time.
Monarch Butterflies
Late January, and the monarch butterflies have found their way home. They’ve entered a hibernation state, huddling together for warmth. But they’re only here at all thanks to one of the most accurate biological clocks found in nature.
These are the pine and oyamel forests, high altitude, about, what, three hours north-west of Mexico City, and one of the few wintering grounds of the monarch butterflies, as you can see. But there is a colony of millions of monarch butterflies somewhere due north of here, so if I head off into the forest then, hopefully, this will just be a taster of what’s to come.
To find the monarch butterflies, I need to get an accurate bearing on them. And to do this I need a timepiece. If you don’t have a compass, how can you tell which direction’s north and which direction’s south? Well, you can use the sun. The Sun rises in the east, sets in the West, and at midday, in the northern hemisphere, it’s due south. But what if it ISN’T midday? Well, there is an old trick, which is to use a watch. See, it’s about three in the afternoon now, and if you line the hour hand of your watch up with the Sun, then, in the northern hemisphere, the line in between the hour hand and 12 o’clock will point due south. Which means north is that way.
For thousands of miles on their way here, the monarch butterflies have faced the same problem. To make their way South, it’s no good simply following the sun. Because, as the day progresses, the sun’s position drifts across the sky. Somehow they have to correct for this.
They use what is called a time-compensated sun compass. They measure the position of the sun every day, using their eyes, but it’s also thought they can measure the position even when it’s cloudy, by using the polarisation of the light. Having locked onto the sun, their brain then corrects for its movement across the sky By using one of nature’s most accurate timepieces. By combining the information from their precise clocks and their eyes, they can navigate due south. That ability to orientate themselves is, I think, one of the most remarkable things I’ve seen.
The biological clocks that have brought the monarch butterflies home are not unique to butterflies. Almost all life shares in these circadian rhythms. They’re an evolutionary consequence of living on a spinning rock.
Our world turns on its axis once every 24 hours, giving us a day. It’s on a billion-kilometre journey around the sun, and each orbit gives us a year. We live inside a celestial clock, one that has been ticking away for over 4.5 billion years. And that’s a full third of the age of the universe.
This is the final ingredient that our home has provided. Time.
Evolutionary Milestones
Take the horse. Like all complex living things, it’s here because our planet has been stable enough for long enough to allow evolution time to play.
The horse is the animal whose family tree we know with the highest precision. So it’s possible to lay out just one unbroken chain of life that stretches back nearly 4 billion years. Animals that are recognisably horselike have been around for a long time – something like 55 million years. You then have to jump quite a lot to something like 225 million years if you want to ask the question, where is the earliest mammal?
And it’s this thing, which looks something like a little shrew. 535 million. This is the point when complex life really began to explode in the oceans. You then have to sweep back a long, long time to find the next evolutionary milestone, arguably the most important milestone – the emergence of the complex cells, these eukaryote. And then, you have to step back a long way in time. You have to step back all the way to here, the emergence of the prokaryotic, the first life form. And so, we have this beautiful long line. We can trace my friend, the horse, and his ancestry back to the events that happened 3.5, 3.6, 3.7 billion years ago on the primordial Earth.
Looking back over that vast sweep of time, you could ask yourself the question, well, do you need 3.5 billion years to go from a simple form of life to something as complex as a horse? Well, the answer to that question is, we don’t know for sure. It seems that you need vast expanses of time, but do you need those big step’s on the simple cell to the complex cell, do you need the gap from the complex cell evolution of multicellular life? We don’t know. We only have one example. There is only one planet where we’ve been able to study the evolution of life, and it’s this one.
And Earth has been an interesting mixture of stability and upheaval. It’s had an environment that’s never completely conspired to wipe out life, but it’s constantly thrown it challenges.
The deep time that our planet has given life has allowed it to grow from a tiny seed of genetic possibility to the planet-wide web of complexity we are part of today. It has given us all the biological clocks we need to synchronise our lives with the rhythms of the planet on its journey around the sun.
External Links
Biological Clocks – National Sleep Foundation
