Genyornis eggshell

Fragments of Genyornis eggshell are found in recently deflated sand dunes where the birds nested, with morphological features differentiating Genyornis eggshell from those of Dromaius (emu)20. We analysed Genyornis eggshell from nearly 2,000 localities across Australia; none is clearly younger than 50±5 ka, whereas Dromaius eggshell are commonly found in the same regions from >100 ka to the present8. Field studies in ten regions across the continent (Fig. 1) yielded >200 collections that contain variably blackened Genyornis eggshell, frequently blackened at only one end of the fragment, suggestive of irregular heating patterns.

Figure 1: Regions across Australia from which diagnostic burnt Genyornis eggshell were collected. Map of Australia showing the ten regions from which burnt Genyornis eggshell fragments have been collected (solid circles) and one other region where other key samples were collected (open circle). A: Arumpo Station, NSW; GA: Garnpung Station, NSW; D: Lower Darling River, NSW; WL: Wallaroo, SA; PB: Port Broughton, SA; P: Port Augusta region, SA; E: Lake Eyre, SA; Q: Sites on and around Quobba Station, WA; GN: Sites on and around Gnaraloo Station, WA; W: Sites on and around Warroora Station, WA; B: Bullara, Ningaloo, and Cardabia stations, WA. Full size image

Diagnostic patterned burning

To test whether blackened eggshell is diagnostic of high temperatures, we sub-sampled partially blackened fragments for amino-acid analysis. We found that the blackened ends had been heated sufficiently to decompose all amino acids, with decreasing decomposition away from the burnt end, eventually reaching amino-acid concentrations similar to those in unburnt fragments (Fig. 2). Such a strong decomposition gradient can only be accomplished if the blackened end was briefly in contact with a localized high-heat-source, likely an ember; rapid amino-acid decomposition requires temperatures ≥500 °C (ref. 21). Graded amino-acid decomposition is apparent in transects across other variably blackened fragments, whereas fully blackened eggshell are devoid of amino acids, consistent with sustained high heat over the entire fragment (Supplementary Figs 1 and 2).

Figure 2: Characterizing blackened Genyornis eggshell. (a) Excavated variably burnt and unusually broken Genyornis eggshell from the Wood Point site, Port Broughton, Spencer Gulf, SA (PB, Fig. 1). (b) Genyornis eggshell fragment from region W (Fig. 1), blackened only at one end, with locations of samples for amino-acid analysis (c). (c) Concentrations of the stable amino acids glutamic acid (Glu), valine (Val) and leucine (Leu) are reduced in direct proportion to the degree of visual blackening, with samples 3–6 exhibiting only slightly lower concentrations than in unheated fragments of the same egg; ±1σ uncertainties (7%) based on duplicate analyses. Unheated fragments of the same egg exhibit 10% inter-eggshell variability for the same amino acids (6 fragments analysed). Less stable amino acids show similar patterns. Transects through other similarly blackened as well as fully blackened fragments are given in Supplementary Figs 1 and 2. Full size image

In most collections with burnt eggshell, we found examples of partially and wholly burnt fragments, some burnt so severely that their natural curvature was flattened, and occasionally reversed, as well as many visually unburnt fragments. These collections often form a tight cluster <3 m in diameter without other eggshell nearby, suggesting the source was a single, or small number of eggs. We conclude that natural wildfire could not produce such steep thermal gradients within and between nearby eggshell fragments, as it requires an untenable combination of circumstances. Rather, these characteristics are most consistent with humans harvesting one or more eggs from a nest, making a fire and presumably cooking the egg. For the same reason that it is possible to boil water in a paper cup over a fire without burning the cup, cooking an egg in a manner that does not cause the egg to explode, will not char the eggshell. Records of traditional Aboriginal cooking of emu eggs describe a relatively slow cooking of the eggs, either wrapped in vegetation or in hot ashes in a hole dug in the ground for that purpose, from which the egg would be removed and rotated or shaken frequently, then repositioned22. After cooking, we presume that eggshell fragments were discarded randomly in and around the fire. We find similarly burnt Dromaius eggshell in unambiguous late Holocene archaeological contexts along with other burnt food debris (Supplementary Fig. 3), and burnt ostrich eggshell is found in similar archaeological settings in Africa23,24.

Burnt Genyornis eggshell fragments are most common in coastal sand dunes of Western Australia (WA). Of 567 collections from four WA regions that contained Genyornis eggshell, 192 included burnt fragments, with much smaller proportions in the Darling (1 of 189 collections) and around Lake Eyre, the driest sector of the continent (1 of 542 collections; Supplementary Data 1). Although the Willandra (A, GA) and Spencer Gulf (WL, PB) regions each had few collections, burnt fragments occurred relatively frequently (2 of 6 and 10 of 23, respectively; Supplementary Fig. 5).

Dating burnt eggshell

To evaluate whether burnt Genyornis eggshell only coincide with a human presence in the landscape, we obtained absolute and relative dates using optically stimulated luminescence (OSL), radiocarbon (14C) and amino-acid racemization (AAR). Seven sites with burnt Genyornis eggshell in a stratigraphic context and eight other stratified sites with unburnt Genyornis eggshell have been dated by OSL (Table 1). The cumulative sum of the individual OSL ages and their uncertainties for collections with burnt eggshell yield an aggregate median age of 47.5 ka and a range from 53.9 to 43.4 ka (Fig. 3a), where the upper and lower bounds correspond to the 16th and 84th percentiles of the aggregate distribution (roughly equivalent to ±1σ in a normal distribution). Eight other collections with Genyornis eggshell that lack burnt fragments, but for which AAR indicates they are among the youngest in each region, have a median OSL age of 51.5±4.9 ka (Table 1).

Table 1 Primary geochronological data for sites with burnt Genyornis eggshell. Full size table

Figure 3: Luminescence and radiocarbon dating of burnt Genyornis eggshell. (a) Cumulative probability age distribution derived by summing the individual OSL Gaussian probability distributions for seven sites with burnt Genyornis eggshell (Table 1). The median age is 47.5 ka, with the light dashed lines representing the 16th and 84th percentiles of the aggregate distribution, similar to ±1σ in a Gaussian distribution. (b) Calibrated age ranges (±1σ) for 11 14C dates on unburnt Genyornis eggshell associated with burnt fragments (Table 1), omitting the two dates from Warroora (W, Fig. 1) that are significantly younger than OSL dates from the same sites (no overlap at ±1σ; Table 1). Vertical heavy dashed line represents the age that most closely satisfies the 14C dates (with one exception), suggesting that burnt Genyornis eggshell is unlikely to be younger than 47.5 ka. Unshaded region in a represents the most likely age range for burnt Genyornis eggshell, although a range including the lightly shaded region cannot be confidently excluded. 14C dates are plotted with increasing minimum age upward; letters refer to regions located on Fig. 1. Full size image

Genyornis eggshell were dated directly by 14C using accelerator mass spectrometry following rigorous pretreatment. However, as in other carbonate media from terrestrial settings, eggshell calcite is subject to slow diffusion of younger carbon from its surroundings. For samples older than 40 ka, small amounts of young carbon will result in apparent ages significantly younger than their true age25,26. For example, we obtained finite 14C ages <46 ka on Genyornis eggshell from two collections dated ≥70 ka by OSL and/or AAR (Table 1), despite >50 ka background dates in standards. This demonstration of exchange with younger carbon suggests that all 14C dates >40 ka should be regarded as minimum ages. Unburnt Genyornis eggshell from 13 collections with burnt fragments, including all regions in Fig. 1, were dated by 14C (Table 1). Calibrated 14C ages for two of the Warroora sites are significantly younger than their corresponding OSL ages (no overlap at ±1σ; Table 1) and are considered anomalously young as a result of carbon exchange. For the remaining 11 samples, all but 1 have minimum calibrated 14C ages ≥44 ka. Considered collectively, calibrated ±1σ age ranges for the nine finite and two non-finite ages constrain a likely minimum calendar age for burnt Genyornis to 47.5 ka (Fig. 3b), although somewhat younger ages cannot be conclusively ruled out. These dates refine earlier estimates for Genyornis extinction of 50±5 ka (ref. 8) to 47.5±2.5 ka.

Eggshell relative age is constrained by AAR in intracrystalline protein residues27 isolated from physically and chemically cleaned Genyornis eggshell (n=3,877)28. The amino acid isoleucine epimerizes to its non-protein diastereomer alloisoleucine at a rate dependent on temperature, with their ratio (A/I) reflecting time and the effective diagenetic temperature for each sample. For collections buried ≥2 m and not subjected to any other heat source, effective diagenetic temperature is set by the integrated mean annual temperature (MAT) since the egg was laid28. However, the extra energy imparted to fire-heated eggshell accelerates racemization. To minimize this effect, we select only visually unburnt fragments for AAR analysis, and we analyse multiple fragments from each collection containing burnt fragments. Yet, even visually unburnt fragments often exhibit variably high A/I values, presumably because they were heated at temperatures sufficient to accelerate racemization but too low to create black carbon. Cooking temperatures ≤100 °C for <1 h will not blacken eggshell or significantly raise A/I, whereas brief transient heating >150 °C but well below 500 °C accelerates racemization without blackening the eggshell. To prevent unblackened but fire-heated samples from biasing our interpretations, we screen our results, limiting summary plots to those collections for which the two (or more) lowest A/I differ by no more than 0.02 A/I units, thereby excluding heated (higher) A/I values; for a few collections, we used the lowest A/I if that was the only analysis within 1 σ of the region’s mean A/I. We measured A/I in over 550 Genyornis eggshell fragments from 84 collections that also contain burnt fragments. In all, 63 of the 84 collections met our screening criteria, from which we compute regionally averaged A/I (Supplementary Data 2), and compare those to their corresponding regional MAT in Fig. 4. The close approximation to a simple second-order polynomial regression (r2=0.99) is consistent with the exponential dependence of racemization rate on temperature, based on kinetics derived in ref. 28, and the high correlation coefficient is consistent with a similar age for all 63 collections across all regions (Fig. 1). Age differences between regions >5 ka would in almost all instances significantly lower the correlation coefficient. However, the exact form of the trend line cannot be predicted a priori because of uncertainties in the magnitude of the glacial-age temperature depression for each region. Consequently, the A/I–MAT relation is a necessary, but not sufficient condition to confirm that all collections of burnt Genyornis eggshell are of the same age.

Figure 4: Extent of isoleucine epimerization in Genyornis eggshell compared to their current regional temperatures. Average A/I in Genyornis eggshell from 63 collections with burnt fragments recovered from all ten regions (Fig. 1) for which a secure A/I could be determined, plotted against their current (1960-1990 AD) mean annual temperature (MAT), each with its ±1σ uncertainty (Supplementary Data 2). Letters refer to regions in Fig. 1, numbers in parenthesis are the number of collections from each region, and the number of different eggshell fragments analysed in each region that contribute to the mean A/I value. The close fit to a simple polynomial regression with a trend towards ever-higher A/I for higher regional MATs, an expected result due to the exponential dependency of racemization rate on temperature, is consistent with, although not in itself proof of, all samples being of the same age. Age differences in excess of 5 ka in almost all cases would result in a significantly poorer polynomial fit. Full size image

Temporal distribution of burnt Dromaius and Genyornis eggshell

To further test whether burnt eggshell is diagnostic of human predation, we utilize the temporal distribution of collections containing burnt Genyornis and Dromaius eggshell derived from AAR analyses. If human predation is the sole cause of variably burnt Genyornis eggshell fragments, then similarly burnt Dromaius eggshell should first appear in the record ∼50 ka, occur continuously to the present, but never occur before human arrival. In our WA collections, where burnt eggshell of both taxa are most common, burnt Dromaius eggshell first appear in sites with AAR indicative of 50±5 ka, indistinguishable within stated uncertainties to the dates associated with burnt Genyornis eggshell, remain frequent in collections through to near-modern time, but are not present in any collection >55 ka in the four WA regions (Fig. 5a) or in any of the other regions. Similarly, none of the Genyornis eggshell collections from WA that predate 50±5 ka contain burnt fragments (Fig. 5b); the increasing percentage of burnt Genyornis eggshell in the lowest three A/I bins is consistent with human predation leading to Genyornis extinction.