Adolescence

Mammalian life stages can be defined in any number of ways, based on behavior and survival (Goodall, 1986), maturation of hard tissues (Schultz, 1956), growth curves (Bogin, 1990), hormone profiles (Molinari and Gasser, 2004), or a combination of the above (Bogin and Smith, 2000). Molinari and Gasser (2004), for example, key three phases of modern human growth to three phases of hormone activity: Infancy, beginning before birth and ending before 1.5-2 years, steered by thyroid hormones; childhood, in which the addition and powerful influence of growth hormones dominate growth, even into initial stages of puberty; and the pubertal phase, in which sex hormones initiate maturation of non-skeletal secondary sexual characters as well as the pubertal growth spurt and simultaneously lead to eventual termination in growth by triggering the closure of epiphyses. "Adolescence" is by most definitions the stage that follows the onset of puberty and lasts until adulthood, which is a period of relative physiological homeostasis (e.g., Bogin and Smith, 2000). Both humans and great apes typically experience puberty, the onset of sexual maturation, years before full somatic maturation, a gap which creates a true and extended adolescence - something which is not the case for all primates (see Smith, 1992). Although humans and great apes share many characteristics of their life cycles, humans mature on a much slower time scale, reaching marker events of hard tissue maturation at ca. 1.8 times the age typical for chimpanzees (Table 10.1).

Maturation of the skeleton and dentition (see below) makes it clear that the Nariokotome youth was somewhere in the adolescent stage of growth and development at death, but

Table 10.1 Age of attainment of maturation events in chimpanzees and humans, and the similar ratio between them (Kerley, 1966; Scheuer and Black, 2000; Liversidge, 2003: 86; see also Smith, 1993, 2000)

Age of attainment (year)

Table 10.1 Age of attainment of maturation events in chimpanzees and humans, and the similar ratio between them (Kerley, 1966; Scheuer and Black, 2000; Liversidge, 2003: 86; see also Smith, 1993, 2000)

Age of attainment (year)

Maturation event

Chimp S

Human S

Ratio (H/C)

Emergence of Mj

3 1/3

5.7

1.7

Emergence of M2

6 1/2

11.4

1.8

Rise in a71 serum

7 1/2

12-13

1.7

testosterone

Elbow begins ossifying

7 1/2

12-13

1.7

Tri-radial plate

8

14-17

1.9

pelvis ossifies

Emergence of M3

10 1/3

18-20

1.8

Shoulder ossification

13 1/2

20

1.5

Life span

55+

100+

1.8

what is less clear, however, is whether the pubertal growth spurt itself existed at his time, 1.5 million years earlier in the evolution of the genus Homo (see Smith, 1993; Tardieu, 1998; Anton and Leigh, 2003). A number of studies have found that the human combination of a protracted slow phase of growth in late childhood followed by a peak or spurt in both weight and linear dimensions appears to be unique even among higher primates (Bogin, 1999; Smith, 1993; see Bogin and Smith, 2000; Hamada and Udono, 2002).

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