Professor Lewis Darnell
INTRODUCTION
A major accomplishment of science has been to lift humankind’s horizons beyond the bounds of our planet, in part by the rather limited space travel achieved so far and more significantly, by observations of the Hubble and the European COROT telescopes. Data has been remarkable. There are many exoplanets, some of which are potentially Earth-like and could possibly harbour life, contemplation of which has led to the new academic subject of Astrobiology. This involves the interdisciplinary study of the origins, early evolution, distribution and future of life in the universe, and if extra-terrestrial life does exist, how it can be detected? Lewis Darnell, of the University of Westminster, is Professor of Science Communication. His interests are centred on Astrobiology, with involvement in the forthcoming 2020 ExoMars program which will deliver a European rover and Russian surface platform to search for signs of life on the Red Planet. The most intelligent life that we are aware of in the universe so far, is our own, and in that respect our evolutionary development can be used as a case study to show how interdependent it has been on planetary processes and how the development of extra-terrestrial life will be similarly constrained by the local planetary system on which it develops. Lewis’s presentation looked at several key moments in the history of humankind and how these were driven by Earth influences.
2. EARTH SYSTEMS SCIENCE- a primer
Earth Science, once considered to be hard rock geology only, now encompasses oceanography, meteorology, glaciology and climatology, not least because all these features (or systems) are inexorably linked together in the development of the planet. Note that Ma=million years and ka=thousand years.
3. MAKING OF HOMO SAPIEN
Homo sapiens, part of the family of great apes, or hominins, evolved in East Africa (see- Fig.1). This part of the continent is on a latitude of rainforest belts of the Congo and the Amazon but surprisingly, it is savannah grassland, not rainforest.
Three major events impacted on East Africa causing a rapidly changing environment which laid a foundation for hominin evolution. Firstly was the tectonic uplift of Himalayas and the Tibetan Plateau, caused by the collision (which continues today) of the Indian and Eurasian plates. The extensive Tibetan plateau drives a huge Asian monsoon (seasonal change in wind) in India and South East Asia, drawing moisture away from East Africa and reducing rainfall.
Secondly, 30Ma ago, a mantle plume beneath East Africa elevated the landscape by over 1,000 m. There was crustal splitting creating the East African Rift valley (see Figs 1 & 2), a North-South fissure from Ethiopia to Mozambique. Development of the rift valley, which assumed its current landscape 5.5-3.7 Ma, consists of a deep valley lined with extensive mountain ridges on both sides (see Fig 2). The effect of the high ridges of the rift and the elevated landscape was that moist winds from the Indian Ocean were forced to rise, discharging their rain at the coast, creating a rain shadow in the rift. Also, moisture from Central African moving eastward was blocked by rift highlands. Plume-induced surface volcanicity formed a series of rocky ridges, easily traversed by hominins, but a barrier to their prey, providing captive prey and yet, protecting hominins from possible intruders.
Fig. 1. Map of East Africa with main modern lake and palaeolake basins
Finally, 3-4 Ma ago, Australia and New Guinea drifted north and constricted the westward flow of warm Pacific water. Instead cold water of the Northern Pacific flowed into the central Indian Ocean, reducing evaporation, further drying East Africa.
Ecosystems were changed considerably. East Africa became mountainous with vegetation ranging from cloud forest to savannah to desert scrub. A consequence of the replacement of forest with savannah led to a proliferation of large herbivores, a source of food for early hominins. These influences were particularly felt 3-4 Ma, the same time as hominin evolution occurred.
Fig. 2. Cross section through East Africa showing the main modern lakes and palaeolake basins
However, changes wrought by geology and the drying of this part of Africa do not fully explain why evolution occurred. The answer lies also in the mechanics of Earth’s orbit, the cyclic variations (Milankovitch cycles) which alter the amount and distribution of solar radiation hitting the Earth’s surface and influencing local climate (see Fig.3). Milankovitch eccentricity and precession cycles drove a variable climate regime in East Africa causing major shifts every~800ka. Evolving hominin intelligence, it is thought, provided adaptability in dealing with these fast moving changes in climate.
Fig 3 The Milankovitch cycles- stretch (eccentricity), tilt and wobble (precession) alter the heat balance on the Earth’s surface
Particular periods of extreme Milankovitch climate variability, were:
2.7-2.5 Ma
1.9-1.7 Ma
1.1-0.9 Ma
Of the known 15 Hominin species, 12 appeared during these variable periods, as did progressively more complex tool technologies. The number of hominin species reached a maximum during the middle period. Homo sapiens is now, surprisingly, the sole survivor of our genus.
The rift valley in East Africa featured many lakes which provided food and water. Some of these are ‘amplifier lakes’, considered a key link between tectonics, rapid local climate fluctuations and hominin migration and evolution. They are so-called because they were very sensitive to precipitation and evaporation rates, a small change in the balance of which, caused the lakes to fill or empty. The filling of amplifying lakes limited settlement space leading to migrations. In consequence, timings of migrations out of East Africa are linked the lake level variations.
Home erectus left Africa 1.8Ma and migrated as far as China. In Europe, Homo erectus eventually evolved into Homo Neanderthalensis, and in Asia, into Homo denisova. Those Homo erectus that remained in East Africa became Homo sapiens, 300-200 ka ago.
Our species, Homo sapiens, dispersed from Africa ~60 ka, having spent longer adapting to the quickly changing climate of East Africa, and encountered species of previous waves. Neanderthals and Denisovans, both original settlers, became extinct by 40 ka. It is a surprise that Neanderthals, well adapted to cold weather and experienced in stone tools should not have survived longer. Perhaps they were killed by Homo sapiens but most likely, outcompeted by the better sapien brain with a talent for social organisation.
4. OUT OF AFRICA BUT CONTINUING CHILDREN OF PLATE TECTONICS
Plate tectonics not only kicked started human evolution, but also surprisingly defined where humanity embarked on building early civilisations. Plate boundaries are notoriously unsafe with frequent earthquakes, volcanic activity and the risk of tsunamis.
Fig 4 Major early civilisations and their proximity to plate boundaries
Yet, overlaying the positions of early civilisations with a tectonic map of plate boundaries (see Fig.4) indicates most are located at plate margins. The regularity of this observation suggests it is unlikely to be due to chance.
Tectonic uplift generates mountain ranges, which together with associated thrust faults provide essential springs of water. Foreland basins, lying adjacent to the uplifted land produce fertile soils, irrigated by rivers flowing off adjacent mountains and depositing rejuvenating sediment. Volcanoes, frequently found at plate boundaries, also produce good, rich soil. Historic trade routes often followed plate boundaries between tectonically sited towns and cities.
The Harappan civilisation opted for the fertile soils of the foreland basin of the Himalayas, irrigated by the big rivers of the Indus and Ganges on the tectonic boundary between the Eurasian and Indian plates. Mesopotamia was founded were the Tigris and Euphrates flow along a subsiding foreland basin pushed down by the Zagros mountains, formed by the subduction of the Arabian plate against the Eurasian. The Assyrian and Persian civilisations also arose on this plate junction. Likewise, the Minoan, Greek, Etruscan and Roman civilisations opted for plate junctions and the South American Mayan and Aztecs also chose to live in similar geological situations.
5. DISPERSION
Ice ages have been a cyclic feature of the planet for the past 2.6Ma, most particularly the Northern hemisphere because of its greater continental land masses, especially in the mid to high latitudes. The last ice age began 80ka and ended 15ka ago. Ice ages are in part due to Milankovitch cycles of stretch, tilt and wobble (see Fig. 3) causing northern hemisphere cooling. In addition, closure of isthmus of Panama cut the latitudinal Atlantic-Pacific connection. Warm ocean water was directed instead along the Gulf Stream taking moisture north, a requisite for ice accumulation in a cooling climate.
The last ice age peak was 23ka. Continental shelves were exposed and the sea-level was 120 metres lower than today permitting intercontinental migration between landmasses.
Fig. 5 The ice age world with the migration paths of Homo sapiens and the approximate ranges of Neanderthals and Denisovians
The timing of the exodus from Africa (see Fig. 5) can be determined by examining mutation rates in both mitochondrial DNA (which traces maternal lineage) and the Y-chromosome, which tracks exclusively through the male line. Mapping variations around the globe indicates when modern humans first arrived in different regions, often defining ancient migration paths. Besides determining arrival on various continents, DNA analysis suggests common ancestors, ‘mitochondrial Eve’ who lived 150ka ago and ‘Y-chromosome Adam’, who existed between 200-150Ka ago. Unusually, humans are very uniform genetically, with the greatest diversity in Africa, evidence we originated there.
The first diaspora of hominins went east to India and South East Asia with an early offshoot to Europe at 45ka. In Asia, there was a split into two groups which travelled either side of the Himalayas, one heading north across Siberia and eventually to America. The second group headed south to South East Asia and Australia. Asia and China were reached around 45-50ka, and Australia by 40ka.
The entry route to America was via the Bearing Strait (30ka) and it was crossed regularly for 20ka, in both ways, by animals and humans. By 11ka, the land masses of Asia and America became separated by rising sea levels.
6.THE AGE OF EXPLORATION (and the Volta do Mor)
The Age of Exploration started on the Iberian Peninsula, a place, at the time, peripheral to existing continental trading routes. The ocean routes that developed were defined by the planetary winds and ocean currents, the habits of which had to be learned by early navigators. Ocean currents are in part driven by the winds and directed by obstructing landmasses, themselves dictated by geology. The key to successful exploration lay in getting to grips with the pattern of these winds and currents.
Fig 6. Atmospheric circulation subdivides into three cells in each hemisphere with a similar pattern in both hemispheres. Besides the bands of wind there are five mayor oceanic gyres (see Fig 8) which flow clockwise in the northern hemisphere and anti-clockwise in the South.
The global wind pattern is determined by the need to move heat from the hotter equatorial regions to the cooler poles. In addition, planetary spin imparts a directional force (Coriolis force) to these winds.
Maximum heating of the globe takes place at equatorial latitudes. Convection occurs, air is diverted poleward on reaching the tropopause. At latitude 300, the air has cooled and subsides, returning to the equator along the surface forming the trade winds. This unit of circulation is known as the Hadley cell. Conversely, at the poles, air is chilled by the icecaps. Cold, cooling air, subsides and moves towards the equator. At latitude 600, it begins to warm and convect, returning aloft to the poles (the Polar cell). In between, is the mid-latitude or Ferrell cell, circulation being driven by energy imparted by the Hadley and Polar cells. The net effect is a predictable pattern of wind over the globe (see- Fig.6).
Early navigators, sailing down the coast of Africa, always kept close to the coast, believing that if they were blown out to sea, they would be unable to return home. In the Atlantic, there are four small archipelagos, the Canaries, Azores, Madeira and Cape Verde, which had been known since antiquity. Portuguese navigators eventually worked out they could access these islands by using the Canary current down the south west African coast and then pick up the North East Trade winds at 300 to deposit them in the Canaries.
The return to Portugal was the innovative key. By heading west (the Volta do Mor, see Fig.7), away from home, navigators would pick up, when north of 300, the South Westerlies for the homeward leg, the Canaries lying conveniently close to where North East Trades meet the South Westerlies. Steering this route would pass Madeira. As sailors ventured further south down the African coast and steered the Volta do Mor, they found the Azores and finally, Cape Verde.
The further reaches of the African coast were similarly explored by other new techniques involving the interaction of winds and currents. In 1497, Vasco da Gama used a large Volta do Mor to pass round the Cape of Good Hope into the Indian Ocean by executing a south west turn into the expanse of the Atlantic, encountering the Brazil current, heading south until he picked up the Westerlies.
Fig 7 The Volta do Mor- key to accessing the North Atlantic archipelagos.
Once round the Cape and onwards to East Africa and India, da Gama experienced the large scale seasonal winds (the Monsoon). These annoyingly blew for long periods alternately north then south, seasonally restricting navigation between Africa and India. The Portuguese eventually established themselves in India (Goa) to be followed by other European nations seeking trade and influence.
In contrast, fellow navigator, Christopher Columbus, headed westward, found America, tried to return, was wrong footed by the winds but found success when he tracked north picking up the Westerlies for the return part of the journey.
7. TOWARDS GLOBALISATION
Fig. 8 Connecting the world- established trade routes exploiting different winds and ocean currents (gyres shown by light-coloured arrows)
Whilst Portugal looked initially to South East Asia, the Spanish looked to America. Early explorations, funded by the enthusiasm of the navigators and the deep pockets of their Royal patrons, were constrained by a knowledge of winds and currents. In 1513, Magellan had first sight of Pacific Ocean across Panama Isthmus. It took several decades to develop an extensive maritime experience of wind and sea and an eventual realisation that Pacific pattern of winds and currents mirrored those in the Atlantic.
Several major global shipping routes developed (see Fig.8), such as the Manilla Galleon route, running between colonies of New Spain in Acapulco (Mexico) and Manilla (Philippines) for 250 years (1565-1815) exchanging silver (from South America) for silks and spices in the Philippines.
Westerly winds across the Pacific delivered galleons to Californian coast (for replenishment, hence many Spanish place-names on this coast). Silver not taken for trade to Asia was carted across Panama to the Atlantic coast for onward transport to Europe.
In the 17th century, a new route to the East Indies was established making use of the band of Westerly winds to carry ships past the Southern tip of Africa. Southern hemisphere westerlies are unobstructed by major land masses and therefore stronger than those in the northern hemisphere. It took 100 years to fully exploit the roaring forties which also avoided the need to wait for monsoon on the old Portuguese route (see Fig 8), and generally was a faster trip to spice islands of Asia. A historical consequence of this new route, the Brouwer route, was that sailors first set their eyes on Australia, and the Dutch discovered Jakarta. Dutch ships ploughing this route, resupplied in South Africa, which is why Afrikaans is spoken in South Africa.
A demand for spices drove the original European exploration. Later, crops grown in India and Asia were transplanted to the New World e.g. sugar to the Caribbean, coffee to Brazil and cotton to North America. A huge demand for labour led to another transcontinental trading system, the triangular Atlantic slave trade route. This linked Europe, Africa and America. Manufactured good left Europe for Africa. Goods were exchanged for slaves for transportation to America where a further exchange brought raw materials back to Europe, ideal for the developing Industrial revolution.
West Central Africa, including the Gold Coast (now Ghana), the Bight of Benin, Bight of Biafra and the Gulf of Guinea proved, due to prevailing current and winds, an ideal stopping off point to load slaves for transhipment to the Americas.
The trade was abolished in 1807, and the American Civil War finally abolished slavery half a century later. In total, 10 million slaves were transported, 40% to Brazil, 40% to Caribbean, 5% to the US and 15% to Spanish America.
Whilst ‘globalisation’ has seemed a relative recent concept, currents and winds have sustained a globalised trading system for considerably longer than most people suspect.
CONCLUSION
Lewis is an accomplished author with several books under his sleeve. ‘The Knowledge- How to Reboot the Earth after an Apocalypse’ looked at global catastrophe, and what we would need to know to reboot civilisation. The content of the book is essentially a manual for humankind.
His latest book is ‘Origins- How the Earth Shaped Human History’ which formed the basis of his presentation and takes a wide-angle look at how the planet, its many systems and processes, have been influential in the human story, building on themes previously mentioned in Jared Diamond and Tim Flannery’s writings. It is highly recommended.
REFERENCES
Flannery, T. (2012) Here on Earth A Twin Biography of the Planet and the Human Race, Penguin ISBN: 978-0-241-95073-9
Diamond, J. (1997) Guns, Germs and Steel- a short history of everybody for the last 13,000 years, Vintage, London ISBN: 978-0-099-30270-8
Maslin, M.A., Brierly, C.M., Milner, A.M., Shultz, S., Trauth, M.H. & Wilson, K.E. (2014) ‘East African climate pulses and early human evolution, Quaternary Science Reviews,
DOI: 10.1016/j.quascirev.2014.06.012 (Open Access)