22. Ogawa, S., Kudo, S., Kitsunai, Y. & Fukuchi, S. Increase in oxytocin secretion at ejaculation in male. Clin Endocrinol (Oxf) 13, 95–97, doi:10.1111/j.1365–2265.1980.tb01027.x (1980).

23. Carmichael, M. S. et al. Plasma oxytocin increases in the human sexual response. J Clin Endocrinol Metab 64, 27–31, doi:10.1210/jcem-64-1-27 (1987).

24. Holt-Lunstad, J., Birmingham, W. A. & Light, K. C. Influence of a «warm touch» support enhancement intervention among married couples on ambulatory blood pressure, oxytocin, alpha amylase, and cortisol. Psychosom Med 70, 976–985, doi:10.1097/PSY.0b013e318187aef7 (2008).

25. Grewen, K. M., Girdler, S. S., Amico, J. & Light, K. C. Effects of partner support on resting oxytocin, cortisol, norepinephrine, and blood pressure before and after warm partner contact. Psychosom Med 67, 531–538, doi:10.1097/01.psy.0000170341.88395.47 (2005).

26. Feldman, R. Oxytocin and social affiliation in humans. Horm Behav 61, 380–391, doi:10.1016/j.yhbeh.2012.01.008 (2012).

27. Zak, P. J., Stanton, A. A. & Ahmadi, S. Oxytocin increases generosity in humans. PLoS One 2, e1128, doi:10.1371/journal.pone.0001128 (2007).

28. Kosfeld, M., Heinrichs, M., Zak, P. J., Fischbacher, U. & Fehr, E. Oxytocin increases trust in humans. Nature 435, 673–676, doi:10.1038/nature03701 (2005).

29. Domes, G., Heinrichs, M., Michel, A., Berger, C. & Herpertz, S. C. Oxytocin improves «mind-reading» in humans. Biol Psychiatry 61, 731–733, doi:10.1016/j.biopsych.2006.07.015 (2007).

30. Guastella, A. J. et al. Intranasal oxytocin improves emotion recognition for youth with autism spectrum disorders. Biol Psychiatry 67, 692–694, doi:10.1016/j.biopsych.2009.09.020 (2010).

31. Guastella, A. J., Mitchell, P. B. & Dadds, M. R. Oxytocin increases gaze to the eye region of human faces. Biol Psychiatry 63, 3–5, doi:10.1016/j.biopsych.2007.06.026 (2008).

32. Rosenzweig, M. R., Breedlove, S. M. & Watson, N. V. Biological psychology: An introduction to behavioral and cognitive neuroscience, 4th edn (Sinauer Associates, 2005).

33. Herculano-Houzel, S. et al. The elephant brain in numbers. Front Neuroanat 8, 46, doi:10.3389/fnana.2014.00046 (2014).

34. Herculano-Houzel, S. The human brain in numbers: a linearly scaled-up primate brain. Front Hum Neurosci 3, 31, doi:10.3389/neuro.09.031.2009 (2009).

35. Herculano-Houzel, S. The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost. Proc Natl Acad Sci USA 109 Suppl 1, 10661–10668, doi:10.1073/pnas.1201895109 (2012).

36. Dunbar, R. I. & Shultz, S. Evolution in the social brain. Science 317, 1344–1347, doi:10.1126/science.1145463 (2007).

37. Dunbar, R. I. M. The social brain hypothesis. Evolutionary Anthropology: Issues, News, and Reviews 6, 178–190, doi:10.1002/(SICI) 1520–6505 (1998) 6:53.0. CO; 2–8 (1998).

38. The World of Air Transport in 2018, (2018).

39. Cartmill, M. in Primate Evolution and Human Origins 14–21 (Routledge, 2017).

40. Bloch, J. I. & Boyer, D. M. Grasping primate origins. Science 298, 1606–1610, doi:10.1126/science.1078249 (2002).

41. Ross, C. F., Hall, M. I. & Heesy, C. P. in Primate origins: Adaptations and evolution 233–256 (Springer, 2007).

42. Heesy, C. P. & Hall, M. I. The nocturnal bottleneck and the evolution of mammalian vision. Brain Behav Evol 75, 195–203, doi:10.1159/000314278 (2010).

43. Jacobs, G. H. Evolution of colour vision in mammals. Philos Trans R Soc Lond B Biol Sci 364, 2957–2967, doi:10.1098/rstb.2009.0039 (2009).

44. Hall, M. I., Kamilar, J. M. & Kirk, E. C. Eye shape and the nocturnal bottleneck of mammals. Proc Biol Sci 279, 4962–4968, doi:10.1098/rspb.2012.2258 (2012).

45. Heesy, C. P. Seeing in stereo: The ecology and evolution of primate binocular vision and stereopsis. Evolutionary Anthropology: Issues, News, and Reviews 18, 21–35, doi:10.1002/evan.20195 (2009).

46. Shamay-Tsoory, S. G. & Abu-Akel, A. The Social Salience Hypothesis of Oxytocin. Biol Psychiatry 79, 194–202, doi:10.1016/j.biopsych.2015.07.020 (2016).

47. De Dreu, C. K., Greer, L. L., Van Kleef, G. A., Shalvi, S. & Handgraaf, M. J. Oxytocin promotes human ethnocentrism. Proc Natl Acad Sci USA 108, 1262–1266, doi:10.1073/pnas.1015316108 (2011).

48. De Dreu, C. K. et al. The neuropeptide oxytocin regulates parochial altruism in intergroup conflict among humans. Science 328, 1408–1411, doi:10.1126/science.1189047 (2010).

49. de Menocal, P. B. African climate change and faunal evolution during the Pliocene – Pleistocene. Earth and Planetary Science Letters 220, 3–24, doi:https://doi.org/10.1016/S0012-821X (04) 00003–2 (2004).

50. Cerling, T. E. et al. Global vegetation change through the Miocene/Pliocene boundary. Nature 389, 153–158, doi:10.1038/38229 (1997).

51. Elton, S. The environmental context of human evolutionary history in Eurasia and Africa. J Anat 212, 377–393, doi:10.1111/j.1469–7580.2008.00872.x (2008).

52. Darwin, C. The descent of man: and selection in relation to sex (J. Murray, 1871).

53. Lovejoy, C. O. Evolution of human walking. Scientific American 259, 118–125 (1988).

54. Milton, K. A hypothesis to explain the role of meat‐eating in human evolution. Evolutionary Anthropology: Issues, News, and Reviews: Issues, News, and Reviews 8, 11–21 (1999).

55. Stanford, C. B. & Bunn, H. T. Meat-eating and human evolution (Oxford University Press, 2001).

56. Carvalho, S. et al. Chimpanzee carrying behaviour and the origins of human bipedality. Curr Biol 22, R180–181, doi:10.1016/j.cub.2012.01.052 (2012).

57. Wheeler, P. E. The evolution of bipedality and loss of functional body hair in hominids. Journal of Human Evolution 13, 91–98, doi:https://doi.org/10.1016/S0047–2484 (84) 80079–2 (1984).

58. Falk, D. Brain evolution in Homo: The «radiator» theory. Behavioral and Brain Sciences 13, 333–344, doi:10.1017/S0140525X00078973 (1990).

59. Langdon, J. H. Umbrella hypotheses and parsimony in human evolution: a critique of the Aquatic Ape Hypothesis. J Hum Evol 33, 479–494, doi:10.1006/jhev.1997.0146 (1997).