Maize Genomics and Genetics 2024, Vol.15, No.2, 80-92 http://cropscipublisher.com/index.php/mgg 83 Isozyme and chloroplast DNA studies have been particularly instrumental in understanding the genetic evolution of maize. These studies confirm that teosinte, particularly Zeamays ssp. parviglumis, is the closest wild relative of domesticated maize. This relationship is evident in the genetic similarities observed in isozyme and chloroplast DNA profiles between teosinte and maize (Doebley, 1990). Moreover, molecular markers such as single nucleotide polymorphisms (SNPs) have been used to trace the genetic diversity and adaptation of maize. SNP analysis helps identify regions of the genome that have undergone selection pressures, providing clues about the traits that were crucial for maize's adaptation to new environments (Aguirre-Liguori et al., 2016; Costa et al., 2022). Phylogenetic studies also reveal the complex interplay between maize and its environment. As maize spread to different regions, it encountered various environmental challenges, leading to genetic adaptations that enhanced its survival and productivity. These adaptations are reflected in the genetic diversity seen across different maize populations, highlighting the importance of genetic research in understanding and leveraging this diversity for future crop improvement (Doebley, 1990). The genetic diversity of maize, its migration patterns traced through genomic studies, and the insights gained from genetic markers and phylogenetic analyses all contribute to a comprehensive understanding of maize's spread and adaptation. These findings not only illuminate the past but also guide future efforts in maize breeding and conservation, ensuring the continued success of this vital crop. 4 Archaeological Evidence for the Spread of Maize 4.1 Key archaeological sites with maize remnants The spread of maize from its origin in southern Mexico is documented through various key archaeological sites that provide substantial evidence of early maize cultivation and usage. One such significant site is Guilá Naquitz Cave in Oaxaca, Mexico, where maize cobs dating back to approximately 6 250 years ago were discovered, marking some of the earliest evidence of domesticated maize (Hart and Lovis, 2013). Another crucial site is San Marcos Cave, also in Oaxaca, where botanical remains including maize cobs and kernels have been dated to around 4 500 years ago. These findings provide insights into the early stages of maize domestication and its initial cultivation practices (Doebley, 1990). In the southwestern United States, the Bat Cave in New Mexico revealed maize cobs dated to about 3 000 years ago, indicating the northward spread of maize agriculture. This site, among others in the region, showcases the adaptation and expansion of maize cultivation into different environmental zones (Hart and Lovis, 2013). 4.2 Radiocarbon dating and other dating methods used in maize studies Radiocarbon dating is a pivotal method for establishing the chronological framework of maize domestication and spread. This technique measures the decay of carbon-14 isotopes in organic material, providing age estimates for archaeological samples. The radiocarbon dating of maize remains from Guilá Naquitz and San Marcos Caves has been instrumental in tracing the timeline of early maize cultivation (Piperno, 2016). Phytolith analysis, another dating method, involves examining silica structures formed in plant cells that remain preserved in soil long after the plants have decayed. Phytoliths specific to maize can be identified and dated, providing additional evidence of maize presence in ancient soils. This method has corroborated radiocarbon dating results in several key sites, enhancing the understanding of maize's spread (Piperno, 2016; Smith, 2017). Stable isotope analysis of carbon and nitrogen in human and animal bones from archaeological sites offers further insights into ancient diets and agricultural practices. This technique can distinguish between different types of plants consumed, including maize, by analyzing the isotopic signatures left in the bones. Studies using stable isotope analysis have confirmed the significant role of maize in ancient diets across various regions (Schoeninger and Moore, 1992).
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