Time was when the model of human evolution went something like this: our ancestors essentially evolved to climb and live in trees, but with the general drying and cooling of the Earth’s climate over the past few million years, the great African forests shrank, to be replaced with the kind of tropical grassland known as savanna. Our ancestors climbed out of the trees, stood upright and learned to walk, or so the story goes.
From this savannah hypothesis springs human bipedalism, hand-eye coordination, large brains, hunting, and the development of civilization as we know it up to and including its apotheosis and zenith, viz. the ability to post pictures of our cats on the internet. Well, it’s a just-so story, of course, and it doesn’t take much digging to expose the underlying themes of progress and ascension of the kind I examine in my Shameless Plug.
The savannah hypothesis has, crucially, withered before the evidence, such as the recent work on stable isotopes in fossil soils showing that the environments in which hominins evolved were, in general, rather open, with no more than about 40% tree cover. The environments in which hominins evolved were neither fully closed, dense forest; nor fully open, steppe-like grassland, but a kind of mixture, a patchwork of different habitats, known as a ‘mosaic’. The degree of woodedness varied over time, and also at several spatial scales.
The word ‘savanna’ has fallen out of favour in the regular discourse – one might say the ‘narrative’ [I dare you: Ed.] – of the study of the environments in which humans evolved, to be replaced by the word ‘mosaic’. We should nevertheless be careful not to replace one woolly concept with another. ‘Mosaic is the new Savanna’ said Kaye E. Reed, tongue firmly in cheek, in a symposium on the question, a highlight of the annual meeting of the American Association of Physical Anthropologists, which was held in the fine city of Calgary, Alberta, whence I have lately returned.
‘Patchiness’ is hardly a new concept to ecologists, who know that habitat heterogeneity has profound effects on species, from their day-to-day behaviour, whether it’s foraging for food or seeking a mate, to their speciation and evolution. In the context of human evolution, however, the concept is made harder to grasp by the added dimension of time, because environments vary in time as well as in space. Reconstructing past environments, therefore, suffers from a number of stern challenges, notably the problem of ‘time averaging’. Ecologists can see an environment in the eternal present, but the sample of a fossil environment from, say, a particular stratum, might be an average of changes over tens or hundreds of thousands of years. And that’s without challenging the assumption that species indicative of particular habitats today might have done the same things in the past. What intrigued me, however, was the concept of a ‘mosaic’ environment itself.
One is a simply a problem of classification. There might well be a habitat that can be described as ‘closed forest’, and another as ‘open grassland’, but things are rarely that simple. Impenetrable stands of trees rarely abut open grassland in this convenient way. Even today, the East African landscape is a mixture of closed forest along the courses of rivers (‘riparian’ forests), mixed woodland, scrub, grassland, semi-desert and swamp, with many gradations between all these. Although the kinds of habitats one sees are largely controlled by geological and climatic variables such as soil type, mean annual temperature and degree of insolation (which is why one doesn’t find icecaps at sea level in East Africa, nor is there much savanna in Antarctica) it’s surprisingly hard to classify the varieties of habitat one sees in a biologically meaningful way.
Another issue – and this is the one that fascinates me – is that of scale. For the environment varies in its patchiness according to one’s size, and how far you are away from it when you’re looking at it. The symposium examined the African habitat at all scales from remote sensing of the entire continent to the heterogeneity – or lack of it – experienced by a small mammal, or a land snail, which mightn’t range over more than a few square metres in a lifetime. You can see the problem rather easily in your own environment, and that’s what I did within hours of arriving home, when the canes croxorum took Crox Minor and me for a body-clock-restoring walk in our usual haunt, Cromer East Beach.
When you look at Cromer East Beach from the air, on the scale of kilometres to hundreds of metres, you’d find something like the picture on the left, a screenshot from Google Earth. North is to the top. You can see Cromer Pier at the northwest corner, and the beach as the yellow strip along the diagonal down towards the southeast. You can see some other habitats too – the sea; the wooded cliffs immediately south of the beach; and inland, the usual English mixture of the urban and suburban (Cromer itself occupies the west of the picture), with patchy fields and woods. At this scale, the beach is a clearly defined environment, something you’d find at the water’s edge. From the colour you’d suspect that Cromer East Beach is sandy. Sand is certainly the impression you’d get on standing on the beach itself, especially at low tide, as you can see in the picture on the right, which reduces the environment from a strip seen from space to something of the order of hundreds to tens of metres, at ground level. In the distance you can see the same wooded cliffs as in the Google Earth image, but look closely – there’s more than just sand here. If you look closely to can make out some distinct habitats within this habitat – smaller worlds not seen or appreciated at larger scales.
One is the breakwater, just discernable in the image above, but quite distinct on the tens-of-metres scale shown in the picture on the left (with Saffron the Jack Russell terrier for scale.) The breakwater is a part of the beach, to be sure, but represents an entirely different community, a patch of something quite distinct, as different from sandy beach as closed rain forest is from prairie or steppe.
Things get really interesting on the scale of tens of centimetres up to a couple of metres, and it’s here you find the most heterogeneity in the beach environment. Here, on the right, is a close-up of the top of one of the uprights of the breakwater. The breakwater is not a single environment, but a collection of habitats, the patchiness dictated by exposure to sun and tide. Some parts are encrusted with barnacles, other parts with wrack, yet others with green algae like angel-hair pasta, and other parts with mixtures of these.
The breakwater is not the only non-sandy environment on the beach. There is quite a lot of shingle, too, some of which creates tidal pools. One the left is a shingle reef at the tens-of-metres scale. As in the breakwater example, this is the right scale for a dog: you might be able to make out Heidi the golden retriever just to the left of the centre of the picture. Like the breakwater, shingle reefs are quite distinct, as habitats, from the sand. The rock pools are plentifully supplied with creatures such as beadlet anemones, small fishes and shrimps which would perish on the sand, though some creatures, such as shore crabs, are found in both. Again, like the breakwater, shingle reefs are themselves patchy. Each of the rock pools one sees at the centimetre scale is itself a tiny planet of life.
Because the sand and the shingle are on the same horizontal level, they tend to intergrade, raising the problem of classification I discussed earlier. On the right is a picture showing sand next to shingle at the metre scale, but if you look more closely, you’ll see gradations of sand with a hint of shingle; shingle with a hint of sand; and even patches in which the stones are more similar in size, and others in which different sizes of stones are mixed together. The fragmentation continues down past the scale of millimetres, and down to the microscopic. On the left is a dead starfish, cast up on the sand. This single organism provides a rich habitat – and mixture of habitats – for microbes and parasites of every kind. And I haven’t even begun to distinguish between the surface of the beach and the environment beneath it, in which variation in oxygen tension creates distinct habitats more suitable for some organisms than others.
This example, from my own backyard, with no need to travel to East Africa or anywhere else, calls to mind Darwin’s metaphor of the Tangled Bank, at the end of the Origin of Species, which is always worth repeating:
It is interesting to contemplate an entangled bank clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us.
How different this picture, derived by the simple means of keeping one’s eyes open, is from the cartoonish division of habitat into classes such as woodland and savanna, showing it up as less an experimental result than the backdrop required by a story of progression and destiny. However, to say that students of human evolution will slide into over-easy use of the term ‘mosaic’ where they might once have used ‘savanna’ is perhaps too harsh a criticism.
The term ‘mosaic’ at least acknowledges the fact that habitats are tangled banks in which evolution happens in the round, and moment by moment, rather than painted scenery against which a drama unfolds. As such, the term ‘mosaic’ is a slippery, protean thing, hard to define, because it is inseparable from the creatures that are evolving, and their interactions with one another. At the end of the symposium, discussant Yohannes Haile-Selassie of the Cleveland Museum of Natural History (who organised the session with Amy L Rector of the Virginia Commonwealth University in Richmond, VA) said that after a whole afternoon of wresting with the concept of the mosaic, we still had got no closer to working out the role of mosaic habitats in human evolution. Which is perhaps as it should be.