As previously suggested10 — and as sup-ported by the identification of enzymes medi-ating NAE biosynthesis and degradation in even elementary forms of life — NAEs are phylogenetically ancient. So it could be that, during evolution, when cannabinoid receptors and PPAR-α were first used as signal transduc-ers, they found endogenous NAEs and used them as activators. Regardless of the identity of their receptors, however, the lipids’ key role in the control of food intake and development seems to have been conserved from simple organisms to humans (Fig. 1). Thus, finding the molecular targets of NAEs in C. elegans

should be made a priority, because these pro-teins might also have been retained in mam-mals, with potential implications for medical research in ageing, obesity and associated metabolic disorders. ■

Luciano De Petrocellis and Vincenzo Di Marzo are in the Endocannabinoid Research Group, Institute of Biomolecular Chemistry and Institute of Cybernetics, Consiglio Nazionale delle Ricerche, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy. e-mail: vincenzo.dimarzo@icb.cnr.it

A R C H A E O L O G Y

The cost of cultivationThere were probably many reasons for the adoption of agriculture by prehistoric human societies. A fresh perspective comes from a quantitative estimate of the relative productivity of farming and foraging.

G R A E M E B A R K E R

The origins of agriculture are among the ‘big questions’ about the human past that archaeologists have grappled

with throughout the history of the discipline. A study by Samuel Bowles, published in Pro-ceedings of the National Academy of Sciences1, shows that cultivating cereals was not just harder work than hunting and gathering, but was probably less productive. So why did foragers become farmers?

For decades, most archaeologists have agreed that the first farming — the Neo-lithic Revolution in Gordon Childe’s famous phrase2 — began soon after the transition from the Pleistocene to the Holocene climatic eras some 12,000 years ago, in a few ‘hearths of domestication’ such as southwest Asia, China and parts of the Americas; and that over the next few thousand years a variety of agricultural systems replaced foraging (hunting and gathering) in almost every corner of the globe. As revolutions go it may have taken sev-eral thousand years, and developed piecemeal across the globe, but it remains one of the most extraordinary transformations in human history3.

So why did most foragers decide to become farmers? Childe emphasized the advantages of farming over for-aging: it gave people a reliable food supply, allowing them to settle down, and this, in combination with the pos-sibilities farming created for producing surplus food, provided the springboard for global population growth and

transformations in social complexity that led within a few millennia to urbanism.

Ethnographic studies in the 1960s dra-matically changed perceptions of farming’s advantages over foraging: even in a marginal environment such as the Kalahari desert, for-agers had a secure food base, and to exploit it worked fewer hours than most farmers need to do in non-industrial societies4. In recent decades, therefore, archaeologists have theo-rized about the processes that might have persuaded foragers to become farmers, even though farming probably meant more work, a

H. B

AR

TON

less varied diet and more disease, to say noth-ing of the social stresses of moving from group food-sharing to household-based production. They have variously proposed triggers such as climate change, population pressure and social competition, or combinations of all three, but they have at least agreed that in most situations farming would have been more productive than foraging.

Bowles1 has added further sauce to the rich mix of theories. His analysis of ethnographic and historic figures for foragers and farmers in traditional societies (that is, using hand tools) suggests that the average productivity from cul-tivating seed plants (he doesn’t deal with tubers) is about three-fifths of the return from foraging wild species. In response to the question ‘why did the first farmers farm?’, he proposes that many foragers might well have been attracted to undertake some small-scale farming because it just meant forgoing those wild plants or animals with the lowest calorific return rate. Indeed, there are many archaeological examples of foragers who engaged in cultivating plants

and/or herding animals for many cen-turies before developing a significant commitment to farming.

But why develop such a commit-ment if the returns from farming were likely to be lower than those from for-aging? Bowles concludes that foragers might have been drawn into farming because becoming more sedentary would have lowered the costs of child rearing and inevitably stimulated pop-ulation growth, and because producing surplus food meant increased pos-sibilities for social competition. Both are arguments that would be familiar to Childe, but they still remain more descriptions of what might have hap-pened than explanations of why people took the decisions they did.

Most theorizing about the ori-gins of agriculture has started from the premise that contemporary or ethnographic examples of mod-ern ‘traditional’ foraging and farm-ing societies are likely guides to how late Pleistocene and early Holocene

Figure 1 | Planting rice in Borneo. Today rice is a staple crop on which millions of people depend. But for many prehistoric societies its role as a high-status, special food may well have been more important than its dietary contribution.

1. Fontana, L., Partridge, L. & Longo, V. D. Science 328, 321–326 (2010).

2. Lucanic, M. et al. Nature 473, 226–229 (2011).3. Devane, W. A. et al. Science 258, 1946–1949

(1992).4. Matias, I., Bisogno, T. & Di Marzo, V. Int. J. Obes. 30,

S7–S12 (2006). 5. Fu, J. et al. Nature 425, 90–93 (2003).6. Berdyshev, E. V. et al. World Rev. Nutr. Diet. 88,

207–214 (2001).7. Kilaru, A. et al. Chem. Biodivers. 4, 1933–1955

(2007). 8. Elphick, M. R. & Egertová, M. Handb. Exp. Pharmacol.

168, 283–297 (2005). 9. Pertwee, R. G. et al. Pharmacol. Rev. 62, 588–631

(2010). 10. McPartland, J. M., Norris, R. W. & Kilpatrick, C. W.

Gene 397, 126–135 (2007).

1 2 M A Y 2 0 1 1 | V O L 4 7 3 | N A T U R E | 1 6 3

NEWS & VIEWS RESEARCH

© 2011 Macmillan Publishers Limited. All rights reserved

Q U A N T U M P H Y S I C S

Keep your feet on the groundSome complex problems in physics can be recast as finding the ground state of an interacting quantum system. Not getting excited along the way can be the challenging part. See Letter p.194

W I L L I A M D . O L I V E R

Information processing based on quantum-mechanical interactions has the potential to yield a new and powerful set of computa-

tional tools and capabilities. In essence, replac-ing the classical bits of information in today’s electronic devices with quantum bits (qubits) enables a computation to proceed on the basis of the rules of quantum mechanics. As one might surmise, however, if the quantum nature of this computational evolution were somehow corrupted along the way, the entire computa-tion would be brought into question. On page 194 of this issue, Johnson et al.1 demonstrate that the evolution of a superconducting sys-tem of eight qubits is indeed consistent with the principles of quantum mechanics when a particular protocol called quantum annealing is implemented.

The solution for certain, challenging, opti-mization problems can be determined from the ground (minimal-energy) state of a system of interacting spins. Finding this ground state is itself a computationally demanding problem, and a form of adiabatic quantum computation called quantum annealing is one proposed means to do it2,3. Conceptually, an adiabatic process works as follows. The system starts in a known ground state with the interactions between neighbouring spins effectively turned off. The interactions are then slowly turned on such that the system evolves adiabatically — that is, without ever leaving its instantaneous ground state during this evolution2. If the sys-tem is never excited, always remaining in the instantaneous ground state, then it will surely end in the final, desired interacting ground state and we have the answer.

To illustrate how this process unfolds, consider a corrugated egg carton, with the ridges and furrows creating potential wells, and an egg placed in one of these wells.

As the interactions are turned on, the egg carton slowly changes shape (never so fast that it jostles the egg into an adjacent well), and one of the wells becomes deeper than the others; this deeper well is the true ground state. A quantum egg will keep its cool and quantum mechanically tunnel through the ridges to reach this global minimum, whereas a classical thermal egg must resort to jumping excitedly over them. In the language of quantum anneal-ing, the system continuously seeks the lowest-energy configuration by means of quantum tunnelling mediated by quantum fluctuations. This process should be distinguished from classical thermal annealing, in which thermal fluctuations lead the way.

In their study, Johnson et al.1 employ a one-dimensional Ising spin system, a chain of eight spins which can interact with one another in a nearest-neighbour pairwise manner. The spins are realized with superconducting flux qubits4, manufactured artificial spins whose values ‘spin-up’ and ‘spin-down’ correspond to clockwise and counter-clockwise circu-lating superconducting currents, which are respectively associated with the left and right wells of the qubit’s potential energy diagram (Fig. 1). The authors went to great lengths to make the various qubit parameters tuneable, so that they could carefully calibrate the set-tings for each individual spin, as well as tune the spin–spin coupling strength and its sign (negative for ferromagnetic coupling and positive for anti-ferromagnetic coupling).

To demonstrate the distinction between quantum and thermal annealing, Johnson et al. leveraged ideas from the domain of macro-scopic quantum tunnelling5,6. Consider first a single qubit and its double-well potential. The qubit starts in a symmetrical configuration with a low-potential tunnel barrier and an equal population in both wells — that is, with equal probability for spin-up or spin-down

societies may have functioned in the past, but archaeology increasingly reveals a compli-cated and ambiguous past — another country in which people did things differently. Late Pleistocene foragers in the islands of southeast Asia were burning vegetation to enhance plant productivity 50,000 years ago5,6 and Upper Palaeolithic reindeer hunters had domesti-cated dogs 30,000 years ago7. People in the Near East and China were cultivating wild cereals for thousands of years before the plants acquired the morphologies of modern domes-ticated crops8,9. In North Africa, the wild Barbary sheep was herded 500 years before people acquired domestic sheep and goats10.

There are other examples of experiments in managing animals and plants that failed to survive into modern farming.

In many ‘early farming’ societies, the domes-ticates, and the material culture associated with them (such as sickles in the case of the first cereals in the Near East), were embed-ded in elaborate social and ritual behaviours, suggesting that motivations for acquiring and using new food resources were highly variable and rarely simply a matter of dietary stress or opportunity11,12. In Borneo, for example, rice was used on a small scale for thousands of years by people who relied on hunting and the management of palms and tubers; difficult to grow and high risk, rice only became a food staple there a few centuries ago (Fig. 1). But before that, it almost certainly had a critical social role as a high-status special food with magical properties13.

It is probably illusory to expect some kind of systematic global causality for the first farm-ing14 — people adopted new technologies and foods in many different ways, at many differ-ent timescales, and surely for widely differing reasons. Bowles’s paper1 is further evidence that we should not impose Enlightenment notions of rationality on decision-making by prehistoric foragers. Such decisions had many unintended consequences, often ones that in time changed the social landscape irrevocably and made once-optional additions to forager lifestyles obligatory components of new ways of living. ■

Graeme Barker is in the McDonald Institute for Archaeological Research and the Department of Archaeology, University of Cambridge, Downing Street, Cambridge CB2 3ER, UK.e-mail: gb314@cam.ac.uk

1. Bowles, S. Proc. Natl Acad. Sci. USA 108, 4760–4765 (2011).

2. Childe, V. G. Man Makes Himself (Watts, 1936).3. Barker, G. The Agricultural Revolution in Prehistory:

Why Did Foragers Become Farmers? (Oxford Univ. Press, 2006).

4. Lee, R. B. & DeVore, I. (eds) Man the Hunter (Aldine, 1968).

5. Barker, G. et al. J. Hum. Evol. 52, 243–261 (2007).6. Summerhayes, G. R. et al. Science 330, 78–81

(2010).

7. Germonpré, M. et al. J. Archaeol. Sci. 36, 473–490 (2009).

8. Fuller, D. Q. Ann. Bot. 100, 903–924 (2007).

9. Jones, M. K. & Liu, X. Science 324, 730–731 (2009).10. di Lernia, S. in Before Food Production in North

Africa (eds di Lernia, S. & Manzi, G.) 113–126 (ABACO, 1998).

11. Barton, H. Curr. Anthropol. 50, 673–675 (2009).

12. Finlayson, B. & Warren, G. (eds) Landscapes in Transition (Oxbow, 2010).

13. Barker, G., Hunt, C. & Carlos, J. in Why Cultivate? Anthropological and Archaeological Approaches to Foraging–Farming Transitions in Southeast Asia (eds Barker, G. & Janowski, M.) 59–72 (McDonald Inst.Archaeol. Res., 2011).

14. Fuller, D., Allaby, R. & Stevens, C. World Archaeol. 42, 13–28 (2010).

1 6 4 | N A T U R E | V O L 4 7 3 | 1 2 M A Y 2 0 1 1

NEWS & VIEWSRESEARCH

© 2011 Macmillan Publishers Limited. All rights reserved