Human Universe

There may also be a lower limit on the masses of life-supporting stars, although this is very much an active area of research. Around 80 per cent of the stars in the Milky Way are red dwarfs, and many are known to have solar systems. Red dwarfs have potential lifetimes measured in the trillions of years, so there is no issue with their longevity. Despite their frugal use of fuel, however, red dwarfs tend to be volatile and variable in their light output. Sunspots can reduce their brightness by a factor of two for long periods of time, and violent flares can increase their brightness by a similar factor over time periods of days or even minutes. Planets in orbit around red dwarfs are therefore subject to significant and rapid changes in the amount of light and radiation they receive. Furthermore, because of their low light output, planets must be extremely close to the star if they are to be warm enough for liquid water to exist on the surface, irrespective of the details of their atmospheres. When planets orbit close to stars, they become tidally locked, with one hemisphere permanently facing the star and the other always facing into the darkness of space. We only see one face of our Moon for the same reason – tidal locking is inevitable for moons orbiting close to planets or planets orbiting close to stars. This results in a strange kind of climate for potentially habitable planets around red dwarf stars; there will be regions of permanent day, and regions of permanent night.

I am an aviation geek. I love aircraft. As I set off to film the African scenes for ‘ApeMan SpaceMan’, I noticed that the Ethiopian Airlines Boeing 787 I boarded at London Heathrow, bound for Addis Ababa, registration ET-AOS, was named ‘Lucy’. On the morning of 24 November 1974, Donald Johanson and a team os archaeologists were searching for bone fragments at a site near the Awash River in Ethiopia. The area was known to be a site rich in rare hominid fossils, but on that particular morning, Johanson and his graduate student Tom Gray found little to inspire them. As is often the way in science, however, a dash of serendipity, coupled with an experienced scientist who understands how to increase the chances of receiving its benefits, made a seminal contribution to the understanding of human evolution. Johanson shouldn’t even have been there – he had planned to spend time back at the camp updating his field notes – but as they prepared to leave, Johanson decided to wander over to a previously excavated gully and have one last look. Even though they’d surveyed the area before, this time Johanson’s eye was drawn to something lying partially hidden on the slope. Closer inspection revealed it to be an arm bone and a host of other skeletal fragments – a piece of skull, a thigh bone, vertebra, ribs and jaw all emerged from the ground and, crucially, they were all part of a single female skeleton. The find triggered a three-week excavation, during which every last scrap of fossil AL 288-1 was recovered. They named it Lucy, after track 3, side one, of Sgt. Pepper’s Lonely Hearts Club Band, because this was 1974 and they played it a lot on their tape recorder. ‘Home taping kills music’, they used to say back then, but it also names airliners.

Lucy lived 3.2 million years ago in the open savannah of Ethiopias Afar Depression. Standing just over 1 mete tall and weighing less than 30 kilograms, she would have looked more ape-ike than a human. Her brain was small, about one-third of the size of a modern human’s and not much larger than a chimpanzee’s. The anatomy of her knee, the curve of her spine and the length of her leg bones suggest that Lucy regularly walked upright on two legs, although there are a handful of scientists who would disagree. What is generally agreed upon, however, is that Lucy was a member of the extinct hominin species Australopithecus afarensis, and she was either one of our direct ancestors, or very closely related to them. Her bipedalism was probably an evolutionary adaptation caused by climate change in the Rift. As the number of trees reduced and the landscape became more savannah-like, the arboreal existence of our more distant ancestors became less favoured, and the increasing distances between trees selected for Australopithecus’s upright gait made travel across the ground more efficient.

‘In Who Speaks for Earth?’, the thirteenth chapter of Carl Sagan’s Cosmos, there are two pictures set side by side. One is of footprints covered by volcanic ash 3.7 million years ago near Laetoli, in Tanzania, probably made by an Australopithecus afarensis like Lucy. Some 400,000 kilometres away and 3.7 million years later, another hominin footprint was left in the dust of the Sea of Tranquility. Together, they speak eloquently of our unlikely, magnificent ascent from the Rift Valley to the stars. The remainder of this chapter deals with the 3 million years between Lucy and the Moon. The timescale is ridiculously small: less than a tenth of one per cent of the period of time during which life has existed on Earth. Lucy was little more than an upright chimpanzee; an animal, a genetic survival machine. We bring art, science, literature and meaning to the Earth; we are a world away, and yet separated by the blink of an eye. Our obligation to survive is owed not just to ourselves but also to that Cosmos, ancient and vast, from which we spring,’ wrote Sagan. I’d like to add that we owe it to Lucy as well.