ARTICLE
doi:10.1038/nature10306
Woody cover and hominin environments
in the past 6 million years
Thure E. Cerling1, Jonathan G. Wynn2, Samuel A. Andanje3, Michael I. Bird4, David Kimutai Korir3, Naomi E. Levin5,
William Mace1, Anthony N. Macharia1, Jay Quade6 & Christopher H. Remien1
The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this
issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that
the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore,
we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction
of woody cover since the Late Miocene epoch (about 7 million years ago). 13C/12C ratio data from 1,300 palaeosols at or
adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than 40%
at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern
Africa over the past 6 million years.
There is long-standing debate as to the importance of woody versus
herbaceous cover in the evolution of humans over the past 6 million
years (Myr)1–5. Despite uncertainty as to the nature of the last common
ancestor (LCA) that we share with modern chimpanzees6, it is widely
recognized that the LCA inhabited wooded environments, and that
hominin habitats became less wooded after this divergence some
5–8 Myr ago2,3,5,6. Woody plants provide shade, shelter and food
resources, and as such could have an important role in the evolution
of terrestrial mammals, including humans. For example, shade provided
by this cover may have influenced thermoregulatory and endurance
running adaptations, or nesting and hunting behaviours of early
hominins7–11.
In broader ecosystem-scale terms, the role of ‘savannah’ ecosystems
in hominin evolution remains a subject of debate1–5, although it is
widely recognized that savannahs may have influenced a variety of
hominin adaptations such as bipedalism and dietary adaptations to
novel foods2,3,6–13. Consideration of the role of savannahs in human
evolution began in 192514 with the introduction of what is often
described as the ‘savannah hypothesis’ and continues today in efforts
to reconcile the fossil record of human origins with diverse palaeoenvironmental proxies1–3,5,15. However, an imprecise and often overly
simplistic application of the definition of savannahs hinders progress
in the debate over the timing and nature of their role in human
evolution. To move past this persistent problem, we develop a relationship between the modern carbon isotope ratio in soils and the
amount of woody cover in tropical environments and show that
this can be used as a calibration for estimating woody cover of past
environments. By using this relationship we can focus on the degree to
which habitats were wooded, thereby circumventing any need to
apply a functional definition of savannah to past environments where
only structure can be inferred.
Stable carbon isotopes in palaeosols are a key means of reconstructing ancient environments, particularly those in the tropics in the past
6 Myr or longer. Woody plants, almost all of which use the C3 photosynthetic pathway, would have provided mammals with shade and
shelter from the direct sun16. Tropical grasses, on the other hand, use
the C4 photosynthetic pathway16. Because these two pathways differ in
their discrimination against 13CO2 during photosynthesis, the stable
carbon isotopic composition of soils can be used as a direct indicator
of the fraction of woody cover in tropical ecosystems17–19.
We present a method using stable carbon isotopes to quantify the
fraction of woody cover in tropical ecosystems using new data from
eastern Africa that builds on earlier observations: the fraction of C4
biomass is related to the fraction of woody cover17,18,20. We then apply
this relationship to well-dated fossil sites in the Awash and OmoTurkana basins in Ethiopia and Kenya, which contain sedimentary
archives that are critical to understanding hominin evolution in the
Pliocene and Pleistocene epochs. The in-depth analysis of the relationship between stable carbon isotopes and woody cover in modern
soils permits the reconstruction of the woody cover, or shade, available to hominins in eastern Africa during the past 6 Myr.
Calibrating a ‘palaeo-shade’ proxy
We report results of woody cover measurements and the 13C/12C ratio
(d13C) values of surface horizons from tropical soils for 76 locations in
Kenya, Ethiopia, Malaysia, Botswana20, Zambia20, Australia17 and
Brazil21 (Supplementary Tables 1 and 2). Sites were selected because
of their undisturbed nature (that is, from National Parks or Reserves)
or because of minimal agricultural disturbance. ‘Gap’ samples are from
soils that are not directly under woody cover and ‘canopy’ samples are
from soils directly beneath woody cover. The d13C values of the gap
and the canopy samples are well correlated for the entire data set
(Fig. 1; see also Supplementary Information). In closed settings with
woody cover of approximately .80%, both the canopy and gap areas
show d13C values characteristic of predominantly C3 vegetation17.
Likewise, open settings, with woody cover approximately ,20%, show
d13C values characteristic of predominantly C4 vegetation, both in the
canopy and gap samples. Thus, d13C values in these soils reflect the
amount of woody cover on the timescales of carbon turnover (,10 yr
in the tropics22), and C4-rich ecosystems have a different soil d13C
signature than C3-rich ecosystems whether under a tree canopy or
not. We invert the relationship in Fig. 2 to reconstruct the fraction of
woody cover (fwc) from d13C values of organic matter and soil carbonate in fossil soils (see methods in Supplementary Information):
1
University of Utah, Salt Lake City, Utah 84112, USA. 2University of South Florida, Tampa, Florida 33620, USA. 3Kenya Wildlife Service, PO Box 40241-00100 Nairobi, Kenya. 4James Cook University, PO Box
6811, Cairns QLD 4870, Queensland, Australia. 5Johns Hopkins University, Baltimore, Maryland 21218, USA. 6University of Arizona, Tucson, Arizona 85721, USA.
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RESEARCH ARTICLE
Number of days
–10
Meru
grassland
0
20
–20
Number of days
δ13C canopy
–15
20
–25
1:1 line
–30
–30
–25
–20
–15
30
40
50
60
Meru
woodland
25
0
20
–10
30
40
50
60
40
50
60
Figure 1 | Correlation of d13C between gap and canopy samples for 76
tropical soils used in this study. Best-fit line is using the major axis regression
where d13Ccanopy 5 0.793d13Cgap 26.4; r2 5 0.89.
2
fwc ~ sin {1:06688{0:08538 d13 Com
20
30
Daily high surface ground temperature (°C)
Figure 3 | Surface soil temperatures from soil temperature profiles.
Calculated maximum daily soil-surface temperatures for a 12-month interval
for forest, woodland and grassland sites in the Meru National Park region,
Kenya (see Supplementary Tables 1 and 2).
oxygenase activity relative to carboxylase activity. Humidity and soil
moisture are also higher in shaded areas. Thus in the areas with higher
shade cover due to a woody canopy, C3 photosynthesis may be
favoured even in the gap areas. Conversely, C4 photosynthesis in open
areas is favoured by lower daily-integrated shade where there is higher
temperature, higher light intensity, with lower relative humidity and
soil moisture. Taken together, the abundance of C3 biomass in gaps in
C3-dominated ecosystems, and the abundance of C4 biomass under the
canopy in C4-dominated portions of the landscapes, is related to the
light intensity and the surface ground temperature, and their effects on
photorespiration, humidity and soil moisture.
A structural classification for palaeo-vegetation
–10
–15
δ13C (SOM)
Meru
forest
50
0
This relationship is not a simple linear mixing line between C3 plants
(ca. -24 to -35%) and C4 plants (approximately 211% to 214%;
ref. 16) because C3 herbaceous plants may occur in both open and
closed areas, and canopy areas may contain both C3 trees and both
mixed C3–C4 understory.
Shade provided by woody cover also affects soil temperatures and
the proportion of C3 and C4 biomass in the herbaceous understory.
Figure 3 shows maximum daily soil surface temperature for three sites
in Meru National Park as calculated from sub-surface soil-temperature
loggers for the period June 2009 through to May 2010. Soil ground
surface temperatures varied between 25 uC and 35 uC in riparian forest,
between 28 uC and 48 uC in woodland, and between 30 uC and 60 uC in
grassland. Over a 1-year period, 130 days had daily maximum ground
surface temperatures exceeding 45 uC in the grassland (unshaded)
environment. C4 photosynthesis is an adaption to both high leaf
temperature and low atmospheric CO2 (ref. 16); C4 photosynthesis
at higher leaf temperatures is favoured over C3 photosynthesis
because of increasing photorespiration in C3 plants related to increased
–20
–25
–30
0.00
Number of days
δ13C gap
0.50
Fraction woody cover
1.00
Figure 2 | Woody cover and soil d13C for 76 tropical soils used in this study.
Canopy-weighted d13C values from Supplementary Table 1 have been
corrected for the Suess effect (to 1,750)43,44, assuming a residence time for
carbon in soils to be 10 years22. Ordinary linear regression was carried out on
arcsinesquare-root-transformed
values of fractional woody cover
pffiffiffiffiffiffi 45
(arcsin pffiffiffiffiffi
fwcffi ) ; the dashed line is the OLR function d13C 5 29.02
arcsin fwc 214.49 (r2 5 0.77). SOM, soil organic matter.
It is useful to define formal terminology of vegetation structural
categories like ‘forest’, ‘woodland’, ‘grassland’ before proceeding with
our reconstruction of ancient vegetation. Because of the strong relationship between d13C and fwc (Fig. 2), we adopt a vegetation classification system that is based primarily on woody cover (the United Nations
Educational, Scientific and Cultural Organization (UNESCO) classification of African vegetation23). The principal vegetation types are:
(1) forest: a continuous stand of trees at least 10-m tall with interlocking crowns. (2) Woodland/bushland/thicket/shrubland: where woodland is an open stand of trees at least 8-m tall with woody cover .40%
and a field layer dominated by grasses; bushland is an open stand of
bushes usually between 3- and 8-m tall with woody cover .40%;
thicket is a closed stand of bushes and climbers usually between 3and 8-m tall; and shrubland is an open or closed stand of shrubs up to
2-m tall. (3) Wooded grassland: land covered with grasses and other
herbs, with woody cover between 10% and 40%. (4) Grassland: land
covered with grasses and other herbs, with woody cover ,10%. (5)
Desert: arid landscapes with a sparse cover dominated by sandy, stony
or rocky substrate.
Because this classification does not define a boundary between forest
and woodland in terms of woody cover, we will consider that ‘forest’ has
.80% woody cover based on the requirement for ‘interlocking crown
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ARTICLE RESEARCH
canopies’. Our reconstruction method cannot distinguish between
functionally distinct categories such as woodland, bushland and shrubland, which are defined by the height of woody vegetation.
The term savannah suffers from colloquial misuse, and for that
reason is not recognized in the UNESCO classification. Still, a modern
ecological definition of the term savannah is comprehensive and
includes structural, functional and evolutionary aspects24. Because
our focus is on reconstructing the physiognomic structure of
palaeo-vegetation, we use a purely structural definition of savannah:
‘‘mixed tree–grass systems characterized by a discontinuous tree
canopy in a conspicuous grass layer’’ (from ref. 24). This, and other
common usage of the term1–3,11,19,25,26, would include at least ‘wooded
grasslands’ and ‘grasslands’ in the UNESCO structural categories
described above, although woody cover varies significantly within
the savannahs (from about 5–80%; refs 24, 27). Rainfall is widely
recognized as the primary determinant of woody cover along with
tolerance to fire, herbivory and soil fertility18,24,27,28. However, variation
in these primary determinants, such as between regions or continents
or soil types, does not obscure the relationship between fwc and d13C
values of soil organic matter as shown in Fig. 2.
Application to the fossil record
Palaeosols have previously been used to quantify the fraction of C4
biomass in hominin environments25,26,29–31, but the relationship between
d13C and woody cover (Fig. 2) provides a means to estimate the fraction
of woody cover in past environments. Figure 4 shows a summary of
reconstructed woody cover for .1,300 palaeosols associated with
hominin sites in eastern Africa over the past ,6 Myr. More than 70%
of these palaeosols reflect woody cover ,40%. Less than 1% of the
palaeosols are associated with woody cover .70%; therefore, ‘closed’
forests (.80% woody cover) represent a very small fraction of the
environment represented by these palaeosols.
Strata from the Awash Valley and Omo-Turkana Basin span the
period from 7.4 Myr ago to the present, and thus bracket the divergence of the LCA of humans and chimpanzees. Much of the hominin
fossil record derives from these two basins, which are well dated, and
contain abundant palaeosols (Fig. 5 and Supplementary Tables 3–6).
We applied our method to long sequences in these two basins to
reconstruct changes in the woody cover over this period (Fig. 6 and
Supplementary Fig. 1). The record in both basins begins with a period
marked by relatively sparse woody cover in the Late Miocene to early
Pliocene. At Aramis, where Ardipithecus ramidus was found, isotopic
compositions are characteristic of grasslands and wooded grasslands31, whereas more wooded conditions were present during
roughly the same interval for Ar. ramidus-bearing sites at Gona.
Thus, there is strong evidence for open habitats in the Late Miocene
to early Pliocene in the Middle Awash region (approximately
5.7–4.4 Myr ago), as well as at Lothagam (7.4–5.7 Myr ago) in the
Omo-Turkana Basin. These generally open habitats are directly associated with the earliest purported members of the Hominini, and this
fact should be considered in debate regarding potential mechanisms
for novel adaptations of this clade, which include bipedalism, the
14°
δ13C (palaeosol carbonate)
–10
–5
0
12°
Gulf of Aden
Percentile
sh
East African palaeosols
associated with hominin sites
(n = 1,380)
99
Aw
a
99.9
Va
ll
ey
–15
10°
95
90
80
70
50
30
20
10
5
Ethiopia
8°
Omo R.
6°
1
Forest
Wooded
grassland
Grassland
Shungura
Nachukui
4°
Koobi Fora
L.
0.1
Woodland/
bushland/
shrubland
na
rka
Tu
Gona
Lothagam
Number of palaeosols
200
2° N
Hadar
Dikika
Middle Awash
Kanapoi
Kenya
0°
Meru NP
100
Indian Ocean
2° S
4°
Modern soil/woody cover/
temperature sites
0
1.00
0.50
0.0
Fraction woody canopy cover
Figure 4 | Estimated fraction of woody cover based on >1,300 published
analyses of palaeosols from eastern African hominin sites from 6 Myr ago to
present25,26,29–31,46–50. Vegetation classification is from ref. 23. Top: cumulative
frequency of palaeosol values related to fraction of woody cover. Bottom:
histogram of palaeosol values related to fraction of woody cover.
Palaeosol/hominin sites
6°
34° E
36°
38°
40°
42°
44°
46°
Figure 5 | Map with modern soil sites and hominin-fossil bearing localities
in the East African Rift System overlain on GTOPO30 digital elevation
model. Red circles, modern soil sites; yellow triangles, hominin-fossil-bearing
localities.
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RESEARCH ARTICLE
Awash Valley
Forest
Woodland/bushland
/shrubland
Omo-Turkana Busin
Wooded
grassland
Grassland
Forest
Encephalized
Aduma (nc = 4, nom = 3)
nc = 17
0
0.16
0
nO = 2, nT = 9
Daka
1
‘Nariokotome boy’
1.63
nO = 5, nT = 92
1.38
nO = 5, nT = 147
1.61
nO = 17, nT = 94
1.87
‘1470’
Acheulean
nc = 84
2
Gona stone tools
nc = 73
2.58
2.7
5
3.24
nc = 10
3.42
nc = 18
3.6
Australopithecus
nc = 18
Adu-Asa fm.
2.85
nT = 43
3.44
nT = 33
3.60
nT = 14
3.9
Gona (nc = 25)
4.6
3
4
4.2
4.3
Aramis (nc = 35, nom = 50)
‘Ardi’
nT = 4
5
Ardipithecus
Western Margin, M. Awash (nc = 19, nom = 32)
Miocene
nO = 6, nT = 39
‘Selam’
5.5
6
2.33
0.1
Prob.
0
5.7
Probability density
5.7
Nawata formation
Hadar fm.
Pliocene
nc = 124
4.1
4.2
Sagantole fm.
Age (Myr)
4
nO = 10, nT = 43
Oldowan
‘Lucy’
2.94
3
Paranthropus
2
Age (Myr)
nc = 35
Omo group (Koobi Fora, Nachukui, Shungura fms
Pleistocene
Busidima formation
1
Bouri fm. (nc = 4, nom = 4)
Grassland
Koobi Fora/Nachukui fm., E./W. Turkana (nT)
Shungura fm. Omo Valley (nO)
Homo
nc = 29, nom = 2
0.83
Wooded
grassland
Carbonate
Megadont
nc = 22
0.64
Woodland/bushland
/shrubland
Overall median
LCA
0.1
Prob.
Individual medians
nT = 11
0
Carbonate (nc)
Lower Awash Valley (Hadar, Dikika, Gona)
Middle Awash sites (as indicated)
+/–
Organic carbon (nom)
7
6
7
1
0.8
0.6
0.4
0.2
0
Fraction of woody cover (areal basis)
1
0.8
0.6
0.4
0.2
Fraction of woody cover (areal basis)
0
7.4
Figure 6 | Composite record of palaeosol stable isotopic composition from
the Awash Valley, Ethiopia (left) and Omo-Turkana Basin, Kenya and
Ethiopia (right). A hominin phylogram is shown at the centre (adapted from
ref. 32). Stable isotope data are presented as normalized probability density
functions of predicted woody cover determined for palaeosols in a series of
temporal bins defined for each basin. Temporal bins are divided based on
marker tephra in each sedimentary basin (see Supplementary Tables 3 and 4).
The number of pedogenic carbonate analyses (nc) and of organic matter
analyses (nom) are indicated for each temporal bin; the median value of woody
cover for all data from each temporal bin is shown with a narrow white bar.
Data from the Awash Valley include relatively continuous records from
co-adjacent research areas at Hadar, Gona and Dikika, along with data from the
Middle Awash region (Aduma, Daka, Aramis and Western Margin). Because
the Shungura formation of the Omo River Valley preserves different
environments from the remainder of the basin36, data from this region are
shown with probability density functions distinct from the remainder of the
record (Koobi Fora and Nachukui formations, East and West Turkana). The
number of analyses from the Omo Valley (nO) and the Lower Turkana Basin
(nT) are indicated. Hominin species ranges are spread according to their age
distribution (vertical axis) and roughly corresponding to their anatomical
features (horizontal axis)32. Major archaeological innovations of early stone tool
development are also indicated: Oldowan technology, first stone tools, 2.6 Myr
ago; Acheulean technology, 1.7 Myr ago.
advent of megadontia, and diminution of the canine premolar honing
complex5,6,32.
The relatively open conditions of the Late Miocene to early
Pliocene are followed by generally increasing woody cover in the
Middle Pliocene (after about 3.6 Myr ago; Fig. 6). Most palaeosols
during this time interval in these basins suggest ,40–60% woody
cover and limited areas of more open environments. This period of
increased woody cover in the Turkana Basin is in part marked by the
onset of rapid basin sedimentation in a tectonically driven, basin-wide
lake system, surrounded by generally mesic environments33. A Middle
Pliocene peak of woody vegetation in the Awash Valley occurs later
than that in the Omo-Turkana Basin, but is also associated with a
period marked by rapid sedimentation in a tectonically controlled lake
that favoured more wooded settings34. Thus, the Middle Pliocene of
both basins is largely characterized by environments that are more
wooded than those in either the Late Miocene (up to about 5.3 Myr
ago) or the Pleistocene (,1.8 Myr onwards)—a trend consistent with
global biome reconstruction and model-data comparisons that show
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ARTICLE RESEARCH
expansion of woody vegetation across Africa during the middle
Pliocene35. These generally more wooded conditions are coincident
with the earliest clear evidence of bipedalism, and the earliest widely
accepted member of the Hominini, as well as more efficient bipedalism
and megadonty of the genus Australopithecus6,32.
The interval straddling the transition from the Pliocene to
Pleistocene (,3.6–1.4 Myr ago) shows the return of open environments such as wooded grasslands and grasslands in these two basins
(Fig. 6). The extent of open grasslands peaks during the Pleistocene
(,1.8–0.01 Myr ago) and represents the culmination of a longer-term
trend towards diminishing woodlands that started in the Late
Miocene. These developments, however, were not synchronous across
the Omo-Turkana system: more wooded conditions persisted longer
in the Omo Valley (into the Late Pliocene, up to about 1.8 Myr ago)
than in the lower Turkana Basin. The presence of a large, perennial
river draining the highlands of the Omo River basin probably
favoured the more wooded environment apparent in these data36.
Thus, in the environment of the generally open conditions of hot
and dry wooded grasslands in the Lower Turkana Basin, areas closer
to the axial river system had more interconnected and dense woody
cover, nourished by more abundant groundwater and surface water.
After 1.9 Myr ago, environments with .50% woody cover nearly
disappear from the stratigraphic record in the basin, including areas
previously characterized by more wooded conditions, such as the
Omo River valley. Woody cover in the Awash Valley also decreases
after about 2.9 Myr ago, which is coincident with a shift from lacustrine
to fluvial deposition36. This overall trend in both basins may be attributable to coeval global and regional climate change4,15,37–40, although a
pronounced reorganization of fault blocks within the Ethiopian Rift
System, and shift from lacustrine to fluvial deposition in Hadar, Gona
and Dikika areas34, may also have contributed to the decrease in the
woody cover after ,2.9 Myr ago. A lack of abundant woody cover
persists throughout the Awash Valley during the Pleistocene, with
several important sites marked by environments with ,10% woody
cover. This expansion of grasslands across the Pliocene–Pleistocene
transition has been linked to global climate change and major developments in the hominin clade3,4,15,40, such as the more obligate bipedalism
of the genus Homo, increase in encephalization, and reduction in tooth
and gut proportions32.
Discussion
Palaeosol stable carbon isotope data from the Omo-Turkana and
Awash River basins provide a strong basis for reconstructing the ecological context of hominin evolution, reinforcing debate over mechanisms of co-evolutionary ecology. The evolution of early hominins
within African savannahs has been a core principle of the savannah
hypothesis proposed by Raymond Dart to explain the origin of the first
Pliocene hominin found in Africa (ref. 14; see also refs 1, 2). Since then,
debate has evolved from an initial, simplistic ‘habitat specific’ savannah
hypothesis, through more sophisticated notions of co-evolution of
hominins and their environment3,4, to more recent total rejection of
the savannahs as a causal mechanism for the divergence from the
LCA5. Using the long-standing recognition that tropical savannahs
are predominantly mixed tree–C4 grass systems24, the stable isotope
evidence from palaeosols discussed in this paper shows that tropical
savannahs have been present in East African hominin sites for at least
6 Myr; within the range of savannah environments, the open grassland
savannah is much less abundantly represented than the wooded grassland savannah. Thus, within the broadly defined savannah biome,
variation in woody cover may have been significant for hominin morphological and behavioural adaptations.
The evidence for the persistence of open vegetation since our divergence from the LCA supports results from a study that used clumped
isotope abundance from some of the same soil carbonates to reconstruct soil palaeotemperature41, which found evidence for elevated soil
temperatures, and related this to the persistence of open vegetation
throughout this basin during the past 4 Myr. Open areas can have
daily high surface ground temperatures—up to 25 uC higher than in
nearby well-shaded riparian forests or woodlands (Fig. 3). This is
because in shaded areas, incident solar radiation is partially balanced
by latent and sensible heat transfer from the surface area of trees,
whereas in open areas of dry soil, sensible heat flux to the soil predominates. Early hominins would have been affected by this uneven
distribution of heat, and this may have influenced physiological and
behavioural adaptations that occurred since divergence from the
LCA2,7–13.
Our observations of the environment of some of the earliest hominins
do not contradict the longstanding hypothesis that savannahs in Africa
may have had a role in the development of bipedal locomotion, or other
key defining characteristics of hominins post-dating the LCA. If either
species of Ardipithecus (Ar. ramidus or Ar. kadabba) is validated as the
‘‘long-sought potential root species for the Hominidae’’42 then the soil
carbonate data now make it clear that both species were surrounded by
more open environments than Australopithecus, which was more efficiently bipedal and occurred in more wooded environments of both the
Omo-Turkana Basin and the Awash Valley (Fig. 6). Thus, the combined
results from two of the most significant hominid-bearing regions in
eastern Africa leave the savannah hypothesis as a viable scenario for
explaining the context of earliest bipedalism, as well as potentially later
evolutionary innovations within the hominin clade.
METHODS SUMMARY
Soils in Kenya and Ethiopia were selected because of their undisturbed nature
(National Parks or Reserves) or because of minimal agricultural disturbance. Soil
samples (0–5 cm depth) were collected following a protocol17,20 whereby multiple
soils from canopy and gap areas are collected in each site over an area comprising
several hectares. Canopy cover was estimated using several methods as described
in the Supplementary Information.
Stable carbon isotopes were measured on the organic fraction in modern soils
remaining after removal of carbonates on an isotope ratio mass spectrometer
operating in continuous-flow mode after combustion at 1,600 uC in an elemental
analyser. Results are reported in the standard per mil (%) notation:
d13C 5 (Rsample/Rstandard 2 1) 3 1000 using the isotope standard Vienna Pee
Dee Belemnite. Overall d13C values of the sites were estimated using the
canopy-weighted values for gap and canopy samples20. d13C values of modern
soils were corrected for the change in the d13C of the atmosphere43,44, assuming a
10-year residence time for carbon in the surface horizon of tropical soils22. Palaeosol
carbonates were analysed on CO2 produced by reaction with H3PO4 at 90 uC.
The several sections of the Supplementary Methods provide additional details
of the methods used.
Received 27 January; accepted 15 June 2011.
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Supplementary Information is linked to the online version of the paper at
www.nature.com/nature.
Acknowledgements We thank the governments of Kenya and Ethiopia for permission
to conduct this research, and F. H. Brown for discussions. The authors thank Kenya
Wildlife Service and members of the Dikika and Gona Research Projects for support in
the field, and Z. Bedaso for aid with analyses. This research was supported by funding
from the LSB Leakey Foundation and NSF grants BCS 0621542, EAR-0617010,
EAR-0937819 and BCS-0321893.
Author Contributions S.A.A., M.I.B., T.E.C., D.K.K., N.E.L., W.M., J.Q. and C.H.R. designed
the modern soil surveys. T.E.C., M.I.B., A.N.M., W.M. and J.G.W. evaluated the amount of
woody cover. C.H.R. and W.M. analysed the soil temperature data. M.I.B., N.E.L., A.N.M.
and J.G.W. analysed modern soils. N.E.L., J.Q. and J.G.W. contributed new palaeosol
data. T.E.C. and J.G.W. wrote the paper with input from all authors.
Author Information Reprints and permissions information is available at
www.nature.com/reprints. The authors declare no competing financial interests.
Readers are welcome to comment on the online version of this article at
www.nature.com/nature. Correspondence and requests for materials should be
addressed to T.E.C. (thure.cerling@utah.edu).
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