Reflections of Their Homelands—Early Life Enamel Formation Disruption in Nineteenth Century Settlers of Otago, New Zealand

Abstract

Physiological stress during early life can impede development, and signals of this are preserved in nonremodelling tissues such as dental enamel. This article describes nonspecific stress markers in the teeth of European (n = 30) and Southern Chinese (n = 15) adult migrants to New Zealand, and colony‐born children (n = 10) interred in four historic Otago cemeteries (c. 1857–1904). Standard histological methods were used to identify and trace accentuated lines (ALs), indicators of enamel formation disruption. All Chinese adults, 93.3% of European adults, and 90% of New Zealand‐born subadults exhibited AL formation. Chinese adults exhibited the highest mean occurrence of ALs per individual, and patterns of AL formation during early childhood differed in individuals of different ethnic and environmental backgrounds. This variation could be attributed to genetic variation in ameloblast susceptibility to stress, disparities in stability of the childhood environment, and/or sociocultural influences on growth and development. These data illustrate the embodiment of physiological stress during early life in poorer classes during the nineteenth century. While childhood hardship was a potential driver of adult decisions to depart for new lands such as New Zealand, it was not absent in the early lives of children born in the colony.

1. Introduction

The Industrial Revolution (c. 1760–1900 AD) transformed people's domestic and working worlds throughout Britain, mainland Europe, and allied colonial settlements (Mitchell 1988; Wrigley 2018). Class divides grew, and poorer classes experienced deterioration of living, working, and health circumstances in association with increasing industrialisation and urbanisation (e.g., Betsinger and DeWitte 2020; Boyd 2020; Buckberry and Crane‐Kramer 2022; Crane‐Kramer and Buckberry 2023; Dutour, Colombo and Coqueugniot 2021). Migration out of Britain to overseas colonies such as Aotearoa New Zealand grew significantly (Phillips and Hearn 2008). New Zealand was often depicted as a “land of plenty”, and many colonial settlers arrived seeking better prospects for themselves and future generations (Phillips and Hearn 2008; Andrews 2009; Ballantyne 2012).

This analysis builds upon previous research that highlighted diverse patterns of childhood physiological stress in early Otago settlers from three colonial cemeteries (King et al. 2022; Kavale‐Henderson et al. 2024), potentially stemming from differences in underlying social and environmental influences. These new analyses incorporate nine individuals from a fourth contemporaneous cemetery site alongside the expansion of previously sampled archaeological dental assemblages to broaden and substantiate previously suggested trends (Kavale‐Henderson et al. 2024).

1.1. Historical and Archaeological Context

Although small numbers of Europeans resided in New Zealand in the eighteenth century, New Zealand was not established as a formal British colony until 1840 (King 2003; Phillips and Hearn 2008; Andrews 2009; Jordan 2010). Subsequently, the foreign population grew rapidly through subsidised migration schemes that targeted young British couples and families of good health and character, favouring those skilled in labour work (Ward 1840; Heale 1842; Phillips and Hearn 2008). These schemes heavily advertised New Zealand as broadly healthy for Europeans, and organised migration remained the primary source of population growth through the 1840s and 1850s (Ward 1840; Heale 1842; Soper 1948). The proclamation of economically viable goldfields at Gabriel's Gully at Tuapeka (now Lawrence) in the Province of Otago on New Zealand's South Island in 1861 stimulated an influx of self‐supported chiefly young male migrants (Gilkison 1930; Ng 2003; Carpenter and Fraser 2016). The goldmining population was chiefly European through the first 5 years of the gold rush until the first arrival of sojourning Chinese goldminers from Victoria, Australia, from late 1865, who comprised 25% of the Tuapeka mining population by 1871 (Ng 1993; Ng 2003; Beattie 2015; Carpenter and Fraser 2016; Davy 2016; Beattie 2017). These Chinese emigrants to Australasia were predominantly peasant farmers from the southern provinces of Guangdong and Fujian, reflecting the difficulties of rural living at this time (Fong 1959; Tai 2005).

Previous bioarchaeological analysis of those who migrated to distant British crownlands such as Australia and New Zealand has indicated these new colonies did not always confer the improvement in wellbeing they may have expected, with evidence of disease, trauma, toxin exposure, and physiological/metabolic disturbances throughout life (Buckley et al. 2020; King et al. 2020; Snoddy et al. 2020; King, Buckley, et al. 2021; King, Petchey, et al. 2021; Gurr et al. 2022; Kavale‐Henderson et al. 2024). Generally, the narratives around the movement of peoples to New Zealand in the nineteenth century centre around premigratory experiences, such as poor living and working conditions, and the subsequent impacts on health that pushed people to depart their “home” countries. However, there are few direct records from these “ordinary” people that detail their early life experiences, and to what extent these may have influenced their decision to migrate. Bioarchaeological methods can mitigate this by exploring experiences of physiological stress in the people of the past through biological evidence from their remains.

1.2. Exploring Physiological Stress Using Biological Evidence

Skeletal change associated with physiological stress occurs in response to stressors, such as illness, malnutrition, or trauma (Selye 1976; Cameron and Bogin 2012; Temple and Goodman 2014). These biological impacts are particularly evident in infants and children due to the demands of growth (Figure S1), and the physical changes associated with survived physiological stress are widely used indicators of early life growth insults (Temple and Goodman 2014; Francis 2018). Stressors can result from adverse changes to an individual's environment, are often interrelated and nuanced, and have been associated with decreased longevity, increased frailty, and premature mortality risk in human populations (Martin et al. 1984; Boldsen 2007; Wyatt et al. 2023). Although a group of individuals may experience similar stressors, the impact of these on the body can be influenced by differences in social and environmental circumstances, alongside genetically moderated responses (Figure S1) (Wood et al. 1992; DeWitte and Stojanowski 2015).

Teeth begin formation in utero and enamel formation is complete in early adolescence (Hillson 2005). Evidence of disruption to growth due to stress is preserved throughout life because hard dental tissues are not remodelled (Moorrees et al. 1963b; Hillson 2005). Teeth also form in a tightly controlled and well‐established sequence, allowing the timing when disruptions occurred to be determined, and comparisons of timing between individuals (Hillson 2005; AlQahtani et al. 2014; McFarlane et al. 2021).

This study explored early life nonspecific markers of physiological stress in 35 migrants and 10 children born in the colony of Otago, New Zealand during the mid‐late nineteenth century through histological analysis of dentition. Temporal patterns of disruptions to dental enamel formation during infancy and childhood in these individuals were considered with regard to ethnic, geographic, and biological backgrounds. This enabled the exploration of potential disparities in childhood experiences of hardship between emigrants of different origins, or between emigrants and children born in New Zealand.

2. Materials and Methods

2.1. Archaeological Sites

Archaeological investigations of four historic Otago cemeteries were undertaken at the request of local and descendant communities to identify “lost” or unmarked graves that lay both within and without modern cemetery boundaries (Figure 1, Table 1) (Petchey et al. 2017; Petchey et al. 2018; Petchey and Buckley 2019). In all cases work commenced after extensive public consultation, accompanied by the requisite archaeological authorities from Heritage New Zealand and disinterment licences issued by the Ministry of Health (Table 1) (Petchey et al. 2017; Petchey et al. 2018; Petchey et al. 2023). All individuals have now been reinterred in their original graves (Kavale‐Henderson et al. 2024), except individuals from Ardrossan Street (AS) Cemetery, who were interred in the Gabriel Street (GS) cemetery as what was the AS cemetery now lies on private land. Summaries of the excavations from St John's Anglican Burial Ground, Milton (SJM), AS and GS have been previously published (Kavale‐Henderson et al. 2024).

FIGURE 1.

Map of New Zealand indicating the modern Otago regional boundary, locations of the towns of Lawrence and Milton, and the former Drybread settlement (grey circles) alongside major cities (black circles).

TABLE 1.

Summary of archaeological sites in which individuals in this study were interred.

Cemetery site	Site abbreviation	Archaeological site code	Geographical location	Dates (AD) in use	Primary 19th C industry in locality	Number of individuals sampled
St John's Anglican Burial Ground	SJM	H45/156	Milton (Tokomairiro Plain), South Otago	c.1860–1890	Service town/farming, sparse gold mining	18 (35 teeth sampled)
Ardrossan Street Cemetery	AS	H44/1135	Lawrence (Tuapeka), Central Otago	c.1861–1864	Gold mining	20 (31 teeth sampled)
Gabriel Street Cemetery (Chinese Section)	GS	H44/1136	Lawrence (Tuapeka), Central Otago	c.1864 onwards	Gold mining	8 (13 teeth sampled)
Drybread Cemetery	DB	G41/630	Manuherikia Valley, Central Otago	c.1860s –currently in use	Farming, gold mining	9 (13 teeth sampled)

Unmarked sections of the Drybread cemetery (DB) were excavated in 2020 and 2021, uncovering 13 graves; however, human remains were preserved in only ten of these burials (Petchey et al. 2023). Still in active use, the excavated burials date from the 1870s to at least 1903 (Petchey et al. 2023). Two graves contained infants, but no dentition remained, and the other four contained European adults with generally good preservation (Petchey et al. 2023). One grave contained no evidence of a burial and/or previous exhumation and appears to have been abandoned prior to completion of the grave cut (Petchey et al. 2023). Six Chinese burials were uncovered in a row now known as the “Chinese area”, with one demonstrating historical exhumation, and these individuals were likely locally engaged in goldmining (Petchey et al. 2023). European migrant families living in the vicinity of Drybread were initially engaged in runholding; however, the population rapidly grew in the mid‐1860s when several local gold diggings were established, remaining active until the 1890s (Petchey et al. 2023).

During the nineteenth century in New Zealand, non‐Europeans were often marginalised, shown here by some Chinese graves being found in discrete areas away from European burials in the GS and DB cemeteries (Petchey et al. 2018; Petchey et al. 2023). Due to the cultural importance placed on a physical return to ancestral lands, many nineteenth century Chinese remains were exhumed for repatriation back to China and this is evident in both GS and DB (Ng 1993; Ip 2003; Petchey et al. 2018; Petchey et al. 2023). The Chinese burials that were not exhumed may represent individuals who no longer held strong ties with China, were from different counties to the organisers of the exhumation schemes, were unable to financially contribute to paid exhumation schemes, or were marginalised by their peers in life and/or death.

2.2. Dental Samples

A total of 92 teeth from 55 individuals from the four historic cemeteries were analysed histologically in this study, representing European and Chinese migrants to Otago, and children born in New Zealand, interred between c.1857–1904 (Table 2) (Petchey et al. 2018; Petchey and Buckley 2022; Petchey et al. 2023). Dental preservation was inconsistent, and many teeth from AS and GS lacked dentine, and all subadult teeth from SJM had minimal or no dentine remaining, leaving only enamel “caps” present (Kavale‐Henderson et al. 2024). All children are likely to have been born in the colony of New Zealand, and all adults have been demonstrated through historical, archaeological, and isotopic evidence to have arrived in New Zealand after childhood. Adults were presumed to have been of European origin if interred in SJM, AS, or DB and were not associated with traditional Chinese grave goods and/or dress, such as braided queues (Table 2). Although individual G1 was interred in the “Chinese section” of GS and was interred with Chinese grave offerings, their mtDNA profile is most encountered in Europeans. A discussion around this conflicting profile has been previously published (King, Buckley, et al. 2021), and as their burial suggests strong affinity with a Chinese identity, they have been included here in the Chinese adult group.

TABLE 2.

Summary of sampled individuals from all cemetery sites.

Individual	Age at death	Sex	Group	Number of teeth sampled	Tooth/ teeth sampled	Biographical notes
SJM1	~5 years	M	Subadult	1	URM1
SJM3b	2–3 years	F	Subadult	3	URc, URm1, and LRm2
SJM4	Mid adult (42?)	Ma	European adult	1	UI1	Identified individual, town doctor, German born, resident in New Zealand for 14 years prior to death in 1874. Cause of death given as alcohol and chloroform abuse. European mDNA haplogroup.
SJM5	7–12 years	F	Subadult	1	LRM2
SJM6	Mid adult (36)	F	European adult	2	LRI2 and LLC	Identified individual, wife of SJM4, resident in New Zealand 21 years prior to death in 1874. Cause of death given as complications from childbirth. European mDNA haplogroup.
SJM8	18 months	F	Subadult	1	URi2
SJM11	Mid adult	M	European adult	4	URC, LRC, LLPM2, and LLM1	Strong osteological evidence for identifying individual, died in mining accident. European mDNA haplogroup.
SJM13	Mid adult?	Ma	European adult	1	LRPM1
SJM16	1.5–2 years	F	Subadult	1	LRm2
SJM18	1–2 years	?	Subadult	1	LLm2
SJM19	9–12 months	M?	Subadult	1	URm1
SJM20a	18 months	F	Subadult	1	LRm2	Double burial with SJM20b.
SJM20b	3 years	F	Subadult	4	URm2, LRm2, ULM1, and LLM1	Double burial with SJM20a.
SJM21	Mid adult (42)	M	European adult	5	URM1, URC, LLC, LRC, and LRPM1	Identified individual, ex‐gold miner. London born, emigrant to Hobart, Australia (1856), then Otago (1861). 12‐month period as an invalid prior to death in 1873. Cause of death given as pneumonic phthisis haemorrhage. European mDNA haplogroup.
SJM22	Adult	M	European adult	2	URM1, ULC
SJM23	Adult	M	European adult	1	LLM1
SJM27	15–18 months	F	Subadult	4	ULi2, LLi2, ULc, and LLm2	Notching in central deciduous incisors, evidence for prenatal enamel formation disruption.
SJM29	Young adult	M	European adult	1	LLC	Evidence for skeletal dysplasia. European mDNA haplogroup.
A1	Adult	M	European adult	2	URM1 and LLC	European mDNA haplogroup.
A4	Adult	M	Chinese adult	1	URM2	Possible Chinese ancestry based on tooth morphology and isotopic evidence.
A5	Adult	M	European adult	1	LRM1
A6	Adult	M	European adult	1	ULM2	European mDNA haplogroup, Oxygen isotopes suggest an origin in Australia, Southern USA, Southern Spain, the Caribbean, or India.
A7	Adult	M	European adult	1	LLM2
A8	Mid adult	M	European adult	1	LRM2?	European mDNA haplogroup.
A9	Adult	M	European adult	1	URM1
A10	Adult	M	European adult	1	LLM1
A12	Adult	M	European adult	2	LRM1 and LRM2
A13	Adult	M	European adult	2	LRM1 and LLM2
A14	Adult	M	European Adult	3	URM1, LLM2, and URM3
A15	Adult	M	European adult	1	LLM1	Pitting defects across crown.
A16	Adult	?	European adult	2	ULI2 and LRM2
A17	Adult	M	European adult	3	URM1, LRM1, and LLM3	European mDNA haplogroup.
A19	Adult	M	Chinese adult	1	URM3	Burnt rice offering in grave suggests individual was most likely Chinese in origin.
A21	Mid adult (30s)	M	European adult	2	ULM1 and ULM3	Partially readable coffin plate. Isotopes indicate a possible highland Scotland origin.
A22	Adult	M	European adult	1	URM1
A23	Adult	M	European adult	2	ULM2 and ULM3
A24	Adult	M	European adult	2	LLI2 and ULPM2	Double burial with very poorly preserved subadult (occipital bone only preserved) laid on body. Long hair preserved.
A25	Adult	M	European adult	1	LLM2
G1	Adult	M	Chinese adult	1	LRM1	Buried with burnt rice offering and hat (traditional Chinese grave goods). European mDNA haplogroup.
G2	Adult	M	Chinese adult	1	ULPM1	Interred in ‘Chinese Section’ of GS Cemetery.
G3	Adult	M	Chinese adult	3	URI2, URM2, and ULM2	Buried with burnt rice offering and hat. East Asian mDNA haplogroup.
G4	Adult	Ma	Chinese adult	2	ULC and LLC	Buried with burnt rice offering.
G7	Adult	M	Chinese adult	1	URM2	Buried with burnt rice offering, coins, and Chinese cone, traditional Chinese queue preserved. East Asian mDNA haplogroup.
G15	Adult	M	Chinese adult	1	LRM2	Traditional Chinese queue preserved. East Asian mDNA haplogroup.
G17	Adult	?	Chinese adult	2	ULC and URM1	Interred in ‘Chinese Section’ of GS Cemetery.
G25	Adult	Ma	Chinese adult	2	LLM1 and URM2	Interred in ‘Chinese Section’ of GS Cemetery.
D1	Mid adult	M	European adult	2	LLC and ULM2	Husband of D2. Skeletal evidence for early syphilitic infection.
D2	Old adult (68)	F	European adult	2	LRM1 and ULM2	Wife of D1. Skeletal evidence for tertiary syphilitic infection.
D3	Old adult (63)	M	European adult	1	LLPM2	Identified through coffin nameplate.
D4	Old adult (72)	M	European adult	3	LLC, LRC and LRPM1	Identified through coffin nameplate. Tooth 33 very poorly preserved and taphonomy precluded observation of enamel microstructure. LEH on tooth 44.
D6	Adult	M	Chinese adult	1	URM2	Likely to be of Chinese ancestry due to burial in Chinese area of Drybread cemetery. Taphonomy precluded observation of enamel microstructure.
D7	Adult	M	Chinese adult	1	ULM1	Likely to be of Chinese ancestry due to burial in Chinese area of Drybread cemetery.
D8	Adult	M	Chinese adult	1	LRM1	Likely to be of Chinese ancestry due to burial in Chinese area of Drybread cemetery. Dental crown malformed (possibly fluorosis).
D11	Adult	M	European adult	1	LLM1
D12	Adult	M	European adult	1	LLC

Note: Data compiled from Buckley et al. (2020), Kavale‐Henderson et al. (2024), King, Petchey, et al. (2021), Petchey et al. (2018, 2023) and Snoddy et al. (2020, 2021). Adult age at death estimates are defined as; young adult (20–34 years), mid adult (35–49 years), older adult (50+ years). Positive identifications were made from the preservation of the writing on the coffin plates. Dental quadrant abbreviations are as follows; Upper right (UR), Upper left (UL), Lower right (LR), and Lower left (LL). Deciduous teeth are notated in lower case, permanent teeth in upper case.

^a Denotes individuals for which no amelogenin peptide analysis data is available, and sex has been estimated through traditional osteological sex estimation methods.

Adult age‐at‐death was estimated based on standard morphological observations of late fusing epiphyses (sternal clavicle and sacrum), pubic symphysis, and auricular morphology (Lovejoy et al. 1985; Brooks and Suchey 1990; Buikstra and Ubelaker 1994; Scheuer et al. 2010), supplemented by dental wear seriation when possible (Petchey and Buckley 2022). Subadult age‐at‐death was estimated from dental growth standards only (Moorrees et al. 1963b, 1963a), due to poor bone preservation. In this study, proteomic sex determination was used to establish the biological sex of individuals in the sample (King et al. 2025). This technique uses sex‐linked isoforms of amelogenin extracted from dental enamel to establish whether individuals have X and Y chromosomes or only X chromosomes (Stewart et al. 2017; Gowland et al. 2021; Gamble et al. 2024), allowing the determination of the sex of subadults as well as adults. For those adult individuals with poorly preserved enamel peptides, standard morphological observations of the cranium and pelvis were used to assess sex (Phenice 1969; Buikstra and Ubelaker 1994). Permanent first molars or canines were preferentially sampled as they begin forming around or shortly after birth (AlQahtani et al. 2014). However, due to variable preservation, high rates of antemortem tooth loss and wear in adults, and lack of formation of teeth in young subadults (<2 years), other tooth types were also sampled (Table 2). To minimise destructive analysis, samples initially sectioned for isotopic analysis rather than histological work were also included. Due to this limitation, alongside the challenge of sectioning often incomplete archaeological teeth, some sections were not made through the apex of the dentine horn.

2.3. Histological Methods

Dental enamel preserves well in the archaeological record due to its high mineral component (96%–97% by weight) (Hillson 2005). Enamel is deposited in sequential layers by ameloblasts, beginning at the cusp tip and terminating at the bottom of the crown (cervix) (Figure 2a), resulting in incremental growth lines being visible in thin sections (Figure 2b) (Hillson 2005). Retzius lines are long‐period incremental lines that represent c. 6–11 days of enamel formation (Figure 2b), aligning with grooves on the outer lateral enamel surface called perikymata (Retzius 1837; Asper 1917; Reid and Ferrell 2006; Mahoney 2008).

FIGURE 2.

(a–c) Diagram of a permanent canine in labio‐lingual cross‐section. (a) Red arrows indicate the ameloblast path in the different zones of enamel (cuspal vs. lateral). (b) Inset of region outlined in image 2a, indicating the general location in which Retzius lines are visible, and their correspondence to perikymata on the enamel surface. The orientation of prism cross‐striations relative to Retzius lines is also highlighted. (c) 200x total magnification of enamel from tooth 26 from individual A21 illustrating the real‐life appearance of prism cross‐striations, composed of one dark and one light band (junction between successively formed cross‐striations is highlighted by white arrows).

Short‐period incremental lines run at right angles to Retzius lines (Figure 2b). These appear as light/dark bands in thin sections, and one light/dark unit represents the circadian (daily) enamel deposition interval (Figure 2c) (Antoine et al. 2009; Aris et al. 2020). These “daily” growth lines are known as prism cross‐striations and are widely used to calculate rates of enamel secretion (Antoine et al. 2009; McFarlane et al. 2021). As different regions of a tooth form at different rates, this daily secretion rate (DSR) varies both within and between teeth (Aris et al. 2020; McFarlane et al. 2021; Aris 2022).

Physiological stress can impede ameloblast function during both enamel secretion and maturation (Nanci and TenCate 2018; Patel et al. 2019; Cardoso et al. 2024). Subsequentially slowed amelogenesis, and thus enamel matrix secretion, results in changes to enamel microstructure that can be differentiated in thin sections from normal incremental markers (Wilson and Shroff 1970; Rose et al. 1978; Rose et al. 1984; FitzGerald and Saunders 2005; Hillson 2014; Aris 2020; Austin et al. 2023). Accentuated lines (ALs) are interpreted as indicators of disrupted enamel formation and are distinguishable from Retzius lines due to their darker appearance, irregular width and continuity across more of the enamel area (Antoine et al. 2019; Goodman and Rose 1990). A prominent AL associated with birth stress, the neonatal line (NNL), forms in the deciduous (primary) dentition and some permanent first molars (Schour 1936; Hillson 2005). The NNL provides a baseline (birth) for when ALs were formed (the age‐at‐AL formation) in days before/after birth (Weber and Eisenmann 1971; Sabel et al. 2008).

Dental samples were prepared for histological analysis using standard methods (Mahoney 2008; Miszkiewicz 2015; Aris 2020), as outlined in Kavale‐Henderson et al. (2024). Histological slides were viewed under transmitted and polarised light on a Nikon Ni‐U microscope with incorporated DS‐Ri2 camera. Using Nikon NIS‐Elements D (v5.10), composite digital slides of the entire tooth section were stitched at 40x total magnification for identification of ALs. Region of interest images were created at 100x and 200x total magnification and comprise the entire enamel width between the dentine‐enamel junction (DEJ) and external surface at approximately mid‐crown (lateral enamel) and just below the dentine horn (cuspal enamel) for DSR calculation. As DSRs vary within zones of enamel, twenty measures of five cross‐striations were taken in the inner, middle, and outer enamel of both lateral and cuspal enamel to estimate local mean DSR values (Figure S2).

Composite images were viewed in FIJI software and ALs were identified and traced. Where more than one tooth per individual was available, an overall individual chronology was produced and ALs were matched across concomitantly forming teeth. Clear ALs were traceable over 75% of the enamel thickness from the DEJ to the outer enamel surface or were chronologically matched either on both sides of the tooth section, and/or to an AL in another tooth forming at the same time, and/or to a surface enamel defect. ALs were classified as potential ALs where they were unable to be traced over this threshold, typically due to taphonomic staining, enamel attrition or postmortem fracturing of enamel (Kierdorf et al. 2021). No hypomineralised lesions were detected on the outer enamel surfaces; however, multiple teeth exhibited macroscopic hypoplastic defects (King et al. 2022). Where possible, direct counts of prism cross‐striations were used to estimate age‐at‐defect, and regional DSRs were employed where cross‐striation clarity was poor or inconsistent. This research has been approved by the University of Otago Human Ethics Committee (25/1244).

2.4. Visualising AL Patterns and Interpreting What They Mean

Physiological, environmental, and cultural influences that impact ameloblast function vary throughout infancy and childhood (Gowland 2015; Bogin et al. 2018; Scheffler and Hermanussen 2018). It is thus important to consider the presence of ALs alongside their pattern of occurrence throughout dental development. To do this, the percentage of individuals with evidence of at least one AL by month of enamel formation was plotted. If stressors affect individuals equally throughout the year, it was expected that the proportion of individuals exhibiting ALs would be broadly similar regardless of when that enamel formed (Figure S3a). However, if exposure to stressors varied throughout the year, or between developmental phases, it was expected that there would be variation in the percentage of individuals with ALs in different months of enamel formation (Figure S3b).

3. Results

3.1. AL Occurrence

ALs were clearly observed in all (15/15) Chinese adults, 93.3% (28/30) of European adults, and 90% (9/10) of New Zealand‐born subadults (Figure S4a–c). The lower left canine from individual D4 and the upper right second molar from individual D6 were badly obscured by taphonomic staining and could not be scored for enamel deposition disruption. Only two European adults (SJM29 and A9) and one subadult (SJM19) did not exhibit evidence of AL formation. In cases where individuals exhibited AL formation, they usually had more than one AL per tooth. Prenatal AL formation was evident in three subadults (SJM18, SJM20b, and SJM27), and one European adult (D11), occurring between 120 and 4 days prior to birth. For individuals from SJM, the estimated age at which ALs formed was associated with previously published records of linear enamel hypoplasia (Table S1) (King et al. 2022).

3.2. Patterns of Enamel Formation Disruption

This section presents patterns of AL formation over the first 5 years of life, corresponding to major developmental periods. When comparing the median proportion of individuals within each group (European adults, Chinese adults, and New Zealand‐born subadults) that exhibited at least one AL per month of enamel formation, a consistent pattern emerged (Table 3). In all developmental periods, this proportion was higher in Chinese adults when compared to European adults (and New Zealand‐born subadults in the first year of life) (Figure 3a–e). AL occurrence appeared to fluctuate more throughout the first 5 years of enamel formation in Chinese adults compared to European adults (Figure 3a–e, Figure S5).

TABLE 3.

The median percentage of individuals that exhibited the formation of at least one accentuated line per month of enamel formation during the major developmental periods.

Age range	Subadults	European adults	Chinese adults
Birth–1 Year	33.3%	31.6%	55.0%
1–2 Years	n/a	17.4%	20.0%
2–3 Years	n/a	14.3%	27.5%
3–4 Years	n/a	17.9%	33.3%
4–5 Years	n/a	7.9%	18.4%

FIGURE 3.

(a–e) Bar charts illustrating the proportion of each group exhibiting AL formation by month of enamel development (approximated to chronological age). The number of individuals per group with enamel forming in each month is presented. Horizontal dashed lines indicate the group median.

Although the median proportion of subadults exhibiting ALs over the first year of life (33.3%) was similar to that of European adults (31.6%), subadults exhibited a wider variance (monthly range 0%–100% of individuals with at least one AL) than European adults (monthly range 16.7%–47.4% of individuals with at least one AL) (Figure 3a). The median proportion of individuals in each group that exhibited ALs also differed temporally when European adults were compared to Chinese adults (Table 3). Both groups exhibited the highest occurrence of ALs in the first year of life, with a reduction in median AL occurrence after this time; however, Chinese adults exhibited a second peak in median AL occurrence between the ages of 2 and 4 years (Table 3).

3.3. A Comparison of AL Formation Chronologies and Weaning Profiles

Previous isotopic studies analysing the weaning behaviour undergone by those in this sample have already been undertaken (King et al. 2022, King et al. 2021). Through analysis of characteristic nitrogen isotope changes in dentinal collagen, these studies have shown that most adults of European ancestry began weaning between 6 and 12 months, with weaning usually complete prior to 2 years of age. In contrast, one European adult (A1) experienced extended weaning, with breastmilk still contributing to the diet until over 3 years of age. Prolonged breastfeeding (≥3 years) was also observed in several individuals of Chinese ancestry (G3, G7, and G25). These extended breastfeeding periods identified isotopically align chronologically with periods of enamel growth disruption observed in this study (Figure 4).

FIGURE 4.

Charts illustrating the occurrence of ALs in individuals that display prolonged breastfeeding isotopic profiles based on dental δ¹³C and δ¹⁵N values. Vertical dotted lines indicate cessation of weaning, vertical red lines illustrate the approximate age at which clear AL formation occurred, and vertical blue lines illustrate the approximate age at which potential ALs formed.

4. Discussion

This study described interruptions to dental enamel formation in migrants and children born in the colony of Otago, New Zealand during the nineteenth century. Almost all individuals in this study exhibited evidence of AL formation, with only two European adults (SJM29 and A9), and one subadult that died in infancy (SJM19) not displaying evidence for enamel formation disruption. Most individuals with ALs exhibited multiple episodes of enamel formation disruption, with ALs visible in teeth forming throughout infancy, childhood, and into adolescence. The high occurrence of ALs in the teeth of people from all four cemeteries was consistent with macroscopic surface evidence for high rates of enamel hypoplasia in these individuals (Petchey and Buckley 2022; King et al. 2022; Kavale‐Henderson et al. 2024).

Globally, the lower classes experienced increasing hardship in association with growth in industrialisation and urbanisation during the nineteenth century. By considering the occurrence of ALs as a proxy of nonspecific physiological stress, insights into the potential impacts of childhood hardship in these Early Otago settlers can be explored. In this study, it was found that over the first 5 years of childhood, European adult AL occurrence per month of enamel formation grouped close to the median, with only one notable peak around 11–13 months of age (Figure 3a,b, Figure S5). This contrasted with Chinese adults, where there were two to three peaks for every year of life (Figure 3a–e). This suggests episodic fluctuations in the prevalence of stressors and/or buffering of these, and therefore potential differences in the stability of the environment in which these individuals were living, and the sociocultural influences that may have buffered or exacerbated the impacts. Section 4.1 discusses the results presented in Sections 3.2 and 3.3 with consideration of key stressors of infants and children, and their impact in relation to key developmental periods, alongside environmental and behavioural factors in the origin countries of early Otago settlers.

4.1. Environmental and Biocultural Stressors and Buffers Impacting Childhood Stress Experiences in Migrants to New Zealand in Their Origin Countries

Food insecurity among the lower classes was globally common during the mid‐nineteenth century (Buer 1926; Floud et al. 1990; Ng 1993; Roberts and Cox 2003; Clayton and Rowbotham 2009; Hao et al. 2021). Rural settlements, such as those in the Pearl River Delta in Southern China, from which many prospective miners departed, experienced inconsistent food resourcing through environmental disasters (e.g., floods) and meagre governmental grain supplementation (Hao et al. 2021). Contrastingly, the shift from rural to predominantly urban settlement in Britain at this time may have increased food security (Woods and Woodward 1984; Clayton and Rowbotham 2008; Betsinger and DeWitte 2020), supported by the establishment of infrastructure such as railways and political legislative changes that lessened restrictions on food access for poorer classes (Woods and Woodward 1984; Williamson 1990; Clayton and Rowbotham 2009; Betsinger and DeWitte 2020; Irwin and Chepeliev 2021).

Due to the rapidly changing energetic requirements of infants and young children, the impact of malnutrition may be increased or reduced at different points of childhood (Guerrant et al. 2008; Scheffler and Hermanussen 2018). Breastfeeding can confer an immune advantage due to the transfer of maternal antibodies, alongside the relatively consistent access to nutrition for the infant (Lunardelli and Peres 2006). Conversely, premature weaning and the introduction of outside food sources may stress an infant, as their nutritional needs are unmet and novel pathogens are introduced (He 1956; Foote and Marriott 2003; Morgan et al. 2004). Severe maternal nutritional stress may also affect the nutritional quality of breastmilk and therefore infant health postnatally, although this is likely to be less severe than in utero effects of maternal stress (Bravi et al. 2016; Adhikari et al. 2022).

During the nineteenth century, weaning practices varied, with European women beginning to wean infants earlier (c. 6–12 months) compared to Chinese women (≥2 years) (Wrigley et al. 1997; Campbell et al. 2002; Lavely 2007; Wolf and Engelen 2007). Through previous isotopic analyses, most European adults examined in this study demonstrate a weaning pattern consistent with this (King et al. 2020), and the peak in AL occurrence seen in European adults in this study, around the age of 1 year, may therefore be reflective of stress associated with weaning (Figure 3a,b). Conversely, prolonged breastfeeding was not uncommon in the nineteenth century when a child was in poor health (Foote and Marriott 2003; Morgan et al. 2004). This is consistent with the observation of extended breastfeeding (≥3 years), in four individuals (A1, G3, G7, and G25) aligning with periods of enamel formation disruption throughout infancy and early childhood (Figures 4, S3b,c).

Potential dissimilarities in the post‐weaning diet of individuals of different geographic origins (Figure 3a–e) may have also contributed to differences in AL occurrence in relation to chronological age. AL occurrence fluctuated more through the first 5 years of life in Chinese adults compared to European adults, and Chinese adults exhibited a second peak in median AL occurrence in Chinese adults between 2 and 4 years of age. The Chinese adults in this study also consistently exhibited a higher median AL occurrence in the first 5 years of life when compared to the European adults.

Alongside food insecurity, high rates of infectious diseases were also common in both Europe and China during the mid‐late nineteenth century (Leung 2008; Hu 2013; Bretelle‐Establet 2019). Typhus and typhoid fevers were endemic to British urban centres, often accompanied by local epidemics (e.g., cholera), and diseases of childhood and infancy such as scarlet fever and diphtheria (peaking 1840–1880), exacerbated by inadequate sanitary measures (Rosen 1973; Luckin 1984; Hamlin 1985; Roccaro et al. 2014; Peckham 2015; Pelling 2022). Similarly, Southern China had high rates of malaria and typhoid, and disease epidemics occurred approximately every 2 years during the Qing dynasty (1644–1911) (Leung 2008; Hu 2013; Bretelle‐Establet 2019). Thus, many of the individuals analysed in this study are likely to have experienced repeated periods of exposure to infectious agents during their early lives, and the potential of this to impact growing bodies is a well‐established (Roberts and Cox 2003; Gluckman and Hanson 2006; Gowland 2015; Newman and Hodson 2021). Individual susceptibility to infectious disease is highly variable, and the impacts of an infectious agent may be exacerbated when a child or infant was already in poor health and/or poorly nourished (Gluckman and Hanson 2006; Guerrant et al. 2008; Gowland 2015; Newman and Hodson 2021; Smith et al. 2023).

4.2. Life in the Colony of New Zealand

Although sold as a “promised land”, New Zealand may not have conferred the advantages in health so commonly advertised to organised settlers. Previous exploration of the factors that may have increased the impact and prevalence of stressors to which colony‐born children were exposed demonstrated that while it was expected that children born in New Zealand would have experienced fewer growth disruption periods than their migrant parents, the two groups exhibited a similar average AL occurrence over the first year of life (Kavale‐Henderson et al. 2024). Here, through the addition of five children that were not previously sampled, and six additional teeth from previously sampled children, the emergent pattern of stress in the colony was corroborated (Figure 3a, Table 3). The lack of clear difference to the parent generation of European adults also reinforces the maintenance of traditional ways of living and doing, actions that had the potential to compromise and/or insufficiently buffer early life stress in these New Zealand‐born children.

4.3. Limitations

Similar stressors such as malnutrition, conflicts affecting food supply chains, and infectious disease epidemic conditions were present in both Europe and China (Woods and Woodward 1984; Morgan 2009; Bretelle‐Establet 2019). The development of enamel defects is influenced by a complex interaction of environmental biosocial and genetic factors (Wright 2023; Cardoso et al. 2024). As genetic variation can impact ameloblast susceptibility to a stressor, and as this study incorporates individuals from different origin populations, this factor cannot be excluded (Alotaibi et al. 2022; Wright 2023). The adult individuals examined in this study were primarily male and of likely low‐to‐middle‐class status, consistent with the economic environment of Otago at the time.

The interpretation of observations of markers of childhood physiological stress in small groups of anonymous individuals interred during the mid‐late nineteenth century in New Zealand is challenging. Nevertheless, results of this study reinforce the recurrent theme of hardship on the lower classes during a period of increasing urbanisation, industrialisation, and globalisation.

5. Conclusions

The nineteenth century was a period of notable change, with deterioration in the living and working conditions of lower classes on a global scale. Perceived poor future prospects and a view to escape hardship for themselves and their children attracted migrants to newly established colonies such as New Zealand. The histological analysis of dentition of both organised settlers and goldrush era migrants to Otago during the mid‐late nineteenth century provided a unique opportunity to explore premigratory experiences of physiological stress. Previous research of migrants and children born in the British colony of New Zealand was built upon to highlight similarities and differences in the childhood experiences of physiological stress in individuals of diverse ethnic, geographic, and biological backgrounds. The data presented in this study reinforces the experience of hardship by many early migrants to Otago, and children growing up in the colony. Chinese adults consistently exhibited higher occurrence and different temporal trends of enamel formation disruption periods over the first 5 years of life compared to European adults. Contrastingly, colony‐born children did not display clear differences in the occurrence of ALs during the first year of life when compared to their parental generation, supporting previously reported observations.

Supporting Information

Additional supporting information can be found online in the Supporting Information section. Supporting Fig. S1: Diagram of the potential impacts on regular body function (homeostasis) when physiological stressors act upon a growing child. Homeostasis, the ideal neutral body state, is indicated by the horizontal dashed line. The outcomes of a stress experience can be influenced by intrinsic and/or extrinsic factors, which act upon the body to move it closer or further to a state of homeostasis. The outcome of focus for studies of survived episodes of childhood stress is highlighted in blue. Supporting Fig. S2: Labio‐lingual cross‐sectional diagram of a canine tooth, indicating the six different enamel regions from which average local DSR rates were calculated. Supporting Fig. S3: Schematics illustrating different potential patterns of accentuated line formation in two hypothetical groups. The median proportion of individuals with ALs for the twelve months of enamel development plotted in both 4a and 4b is around 20%. However, the pattern of AL occurrence at different periods of enamel formation is vastly different. Figure S3a represents a group of individuals in which there is no apparent peak or trough in AL occurrence (and by proxy exposure to stressors and/or buffering of stressors) over a year of enamel formation. Figure S3b represents a group of individuals in which there are clear peaks and troughs in AL occurrence over 1 year of enamel formation, highlighting periods where stressor exposure is relatively high and/or buffering of these stressors was low, and vice versa. Supporting Fig S4: Chronology of accentuated lines by individual, grouped by childhood origin and age‐at‐death; (a) New Zealand‐born subadults, (b) Europe adults, (c) Chinese adults. All subadults are of European ancestry. Horizontal grey bars represent the approximate age at which the preserved enamel was forming. Vertical blue lines indicate the approximate age at formation of potential ALs, vertical red lines indicate the approximate age at formation of ALs that were clearly identified and/or matched on both sides of the tooth section and/or were matched between teeth where overlapping samples were examined. Supporting Fig. S5: Box plot comparing the proportion of individuals in each group exhibiting at least one accentuated line per month of enamel formation over the first 5 years of life. N (months of enamel formation) = 60 for each boxplot. European adult median = 18.7%, European adult mean = 16.7%. Chinese adult median = 31.1%, Chinese adult mean = 29.3%. Supporting Table S1: Estimated Age at Surface Hypoplasia Compared to Temporally Correlated Accentuated Lines (ALs).

Funding

This work was supported by University of Otago (Grant Doctoral Scholarship), Ministry of Business, Innovation and Employment (Grant 21‐UOO‐01), University of Otago Research Grant and Marsden Fund (Grant 18‐UOO‐028).

Conflicts of Interest

The authors report there are no competing interests to declare.

Supporting information

Supplementary Material

Data Availability Statement

Data available within the article or its supplementary materials.