Human Universe (18 page)

Before astrology was consigned to the status of trifling funfair entertainment by science, it was believed that the position of the planets against the distant stars had a profound effect on people’s daily lives. If you don’t know what the stars or planets actually are, this is at least within the bounds of reason, but as our understanding of physics improved, so it became clear that there is no way that the position of a distant planet relative to the fixed stars can have any effect on the behaviour of a human being on the surface of the Earth. The planets can and do affect the Earth’s motion through the solar system over timescales far greater than those of human lifetimes, though, and recent research suggests that long-term changes in Earth’s orientation and orbit may have played a crucial role in hominid evolution.

Polaris is a true giant, almost 50 times the diameter of our sun. It is also a Cepheid variable, one of the valuable standard candles upon which the astronomical distance scale rests. At a distance of only 434 light years, it is both the closest Cepheid and one of the brighter stars in the sky, dominating the constellation Ursa Minor. Polaris also happens to be aligned directly with the Earth’s spin axis, and this special position on the celestial North Pole makes it invaluable to navigators. As the Earth spins on its axis, Polaris sits serenely as all other stars rotate around it. At any point in the northern hemisphere, your latitude is the angle between Polaris and the horizon: zero degrees north at the equator, where Polaris is on the horizon, and 90 degrees north at the Pole where Polaris is directly overhead. As viewed from Oldham, Lancashire, UK, Polaris sits at an angle of 53.54 degrees above the horizon.

Christopher Columbus and Ferdinand Magellan relied on Polaris as they crossed the oceans and explored new worlds. Perhaps more surprisingly, on board Apollo 8 Jim Lovell carried a sextant as a back-up navigational device. Designed by the MIT instrument laboratory in Cambridge, Massachusetts, it may not have looked traditional but it operated in exactly the same manner as the one constructed by instrument maker John Bird in 1757. Polaris was one of Apollo’s key navigational stars. It was paired with Gamma Cassiopeia on Lovell’s charts, which was known in Apollo jargon as ‘Navi’. The name was coined by Gus Grissom on Apollo 1 as a prank – it was his middle name ‘Ivan’ backwards. Two other navigational stars, Gamma Velorum and Iota Ursa Major, were named ‘Regor’ after Roger Chaffee and ‘Dnoces’ after Ed White the ‘Second’. Using the stars for navigation might seem hopelessly old-fashioned, but if you think about it for a moment, you’ll realise that there is no other way that a spacecraft in deep space can orient itself, other than relative to the fixed stars on the celestial sphere.

A spacecraft will often shift its position relative to the stars, but on Earth, things feel different because our orbit around the Sun is relatively stable from year to year. There are wobbles on relatively short timescales associated with changes in the speed of Earth’s rotation, and these lead to the insertion of leap seconds to keep our atomic clocks synchronised with the heavens. Between 1972 and 1979, nine leap seconds had to be inserted, whilst none was needed between the beginning of 1999 and the end of 2005. Earth’s rotation rate is noticeably chaotic when compared to the accuracy of atomic clocks.

The largest short-term contribution to changes in Earth’s rotation comes from the gravitational influence of the Moon, which acts to slow down the rate of spin by around 2.3 milliseconds per century due to friction between the tidal bulges in the oceans and the rotating solid Earth beneath, but there are also longer-term changes. The most pronounced of these is known as axial precession or, more commonly, the precession of the equinoxes. The Earth spins on its axis like a gyroscope, and because it spins, it bulges out at the equator. Because the Earth isn’t a perfect sphere, the gravitational influence of the Sun and Moon exerts a torque on the Earth that causes its spin axis to sweep around in a circle once every 26,000 years. This is not subtle, because the spin axis itself is tilted at 23 degrees relative to the plane of Earth’s orbit, and precession therefore has a large effect on the night sky that was first documented by the Greek astronomer Hipparchus, around 150
BCE
. Precession manifests itself as a shift in the position of the celestial pole relative to the fixed stars. There will come a time in the not too distant future when Polaris will no longer sit above the celestial North Pole as our spin axis traces out a circle in the sky. In about 3000 years’ time, navigators of the future will rely on Gamma Cephei as a back-up for their GPS systems as they sail across the seas of our planet, and in 8000 years it will be the bright star Deneb. The identity of the North Star has altered many times throughout human history. As the Egyptians finished building the Great Pyramid of Giza in 2560
BCE
, Alpha Draconis lay closest to the celestial pole. Two and a half thousand years later, as the Romans did things for us, Kochab, the second-brightest star in Ursa Minor, and its neighbour Pherkad were known as the ‘Guardians of the Pole’. Precession therefore affects navigation, but more importantly it also affects our climate.

The 23-degree tilt of Earth’s spin axis is responsible for the seasons; summer in the northern hemisphere occurs when the North Pole is tilted towards the Sun, leading to constant daylight within the Arctic Circle. Half a year later and the geometry is reversed, with the South Pole receiving 24-hour daylight and the southern hemisphere experiencing summer. Precession alone would have no effect on the climate if the Earth’s orbit were a perfect circle, but it isn’t; it is elliptical, with the Sun at one focus. At the turn of the twenty-first century, it happens to be the case that the Earth is at its closest approach to the Sun (known as perihelion) in January, just after the winter solstice when the North Pole is pointing away from the Sun. This makes northern winters slightly milder than they would otherwise be, because the Earth receives a little bit more solar radiation during the northern winter. In around 10,000 years’ time, however, precession will have carried the Earth’s spin axis around by a half-turn, and it will be the North Pole that points towards the Sun at perihelion, making northern hemisphere summers slightly warmer and winters cooler. The more elliptical the Earth’s orbit, the more pronounced this effect.

This is where things get a little more complicated, but it’s the complication that matters for our story. The planets are significantly further away than the Moon, but also significantly more massive, and their constantly shifting positions induce periodic changes to our orbit over long timescales. Jupiter has the most pronounced effect due to its large mass and relative proximity. The largest of these changes occurs on a timescale of 400,000 years. Picture the Earth’s orbit becoming periodically more elliptical and more circular, stretching back and forth with a period of 400,000 years. This oscillation modulates the effect of precession on the climate; at the times when the Earth’s orbit is at its most elliptical, the changes due to precession will be at their most pronounced. This effect is known as astronomical or orbital forcing of the climate.

There are many such resonances in Earth’s orbit – another important change in the eccentricity of the ellipse occurs every 100,000 years. Furthermore, the tilt of the axis itself swings back and forth between around 22 and 25 degrees on a 41,000-year cycle. The whole solar system is like a giant bell, ringing with many hundreds of harmonics driven by the gravitational interactions between the Sun, planets and moons.

Over many thousands of years, these shifts in the Earth’s orbit and orientation relative to the Sun have led to dramatic changes in climate, and are certainly one of the key mechanisms that drive the Earth into and out of ice ages. It is perhaps obvious that these long-term shifts in climate should have had an effect on the evolution of life; ice ages present a significant challenge to animals and plants and this will provoke an evolutionary response via natural selection. More surprisingly, recent research has suggested a direct link between precession, the 400,000-year eccentricity cycle, and the evolution of early modern humans.

The Great Rift Valley: evocative words that immediately suggest origins. There are many reasons I love visiting Ethiopia. I love the people. I love the food. I love the high-altitude freshness of Addis. I love the mountains and valleys and high plains. I even loved visiting Erta Ale, the legendary shield volcano at the Afar Triple Junction known as the gateway to hell, although I probably won’t do it again. But I also love an idea. It’s impossible to visit this ancient country and not catch a glimpse in your peripheral vision of a chain of ghosts stretching back ten thousand generations, because it is firmly embedded in popular culture that we came from here. Every one of us is related to someone who lived in Ethiopia hundreds of thousands of years ago. It is the Garden of Eden, the place where humanity began. What popular culture has yet to assimilate, however, is the fortuitous and precarious nature of the ascent of man. When I was growing up I remember talk of ‘the missing link’, that elusive fossil that would tie us definitively to our ape-like ancestors. When I started school, DNA sequencing was not yet invented, and Lucy hadn’t been unearthed. Today, we have a significantly more complete view of how Australopithecines like Lucy are related to modern humans, and whilst the details are still debated and new evidence is continually updating the standard model of hominin evolution, it is now possible to tell the broad sweep of the story in some detail.

The members of our human evolutionary family are referred to as hominins. The split between hominins and the ancestor of the chimpanzee occurred at some point before 5 million years ago in Africa, and by 4 million years ago,
Australopithecus afarensis
– Lucy – was present. Their brain size was approximately 500cc, around the same as a chimpanzee and less than one-third of that of a modern human. Around 1.8 million years ago, there was a step change in both brain size and the number of hominin species in the East African Rift. Several species of our genus
Homo
appeared, including
Homo habilis
and
Homo erectus
. They lived for a time alongside other species, including several Australopithecines and a genus known as
Paranthropus
. There are anthropologists who prefer to classify
Paranthropus
as a different species of
Australopithecus
. I make this point not to be confusing, but to highlight an important fact; the study of hominin evolution is a difficult area, and it is not surprising that there are ongoing debates about the classification of 2-million-year-old fossils and DNA sequences. What is important for our story, however, and what nobody disputes, is that there seems to have been a jump in both brain size and the number of species of hominins in the Rift Valley region around 1.8 million years ago. By around 1.4 million years ago, only one of these species had survived –
Homo erectus
– with a brain size of 1000cc. The next milestone is the appearance of
Homo heidelbergensis
around 800,000 years ago.
Homo heidelbergensis
is generally accepted to be the ancestor of
Homo sapiens
and the Neanderthals who lived alongside us in Europe until around 45,000 years ago, and possibly later.
Homo heidelbergensis
represented another jump in brain size, up to around 1400cc, which is close to that of modern humans.