Ecosystems of the recent past, all the way back to the formation of the Earth itself, can be reconstructed by means of palaeoenvironmental proxies. Fossil flora, fauna, and trace fossils can tell us about climate and biodiversity, which can be incorporated with sedimentology, geochemistry and palaeomagnetism. Together, these data allow palaeoecologists to build a detailed model of ancient ecosystems that can help us to understand why life on Earth evolved the way it did. What were the natural selective pressures driving evolution and what relationship do animals have to their environment? Answering these questions will aid in quantifying the current state of biodiversity and how destruction of habitats can impact on species loss.
Palaeoenvironments as drivers of evolution
Organisms are dependent on their physical environment for survival, and factors such as changing climates, geography, fauna and vegetation all stimulate the evolution of unique adaptations by natural selection.
Many species of finches evolved from a common South American mainland ancestor, their beaks adapting to feed in different ecological niches on various islands in the Galápagos archipelago. Some finches have large, blunt beaks that can crack the hard shells of nuts and seeds. Other finches have long, thin beaks that can probe into cactus flowers without the bird being poked by the cactus spines. Still other finches have medium-size beaks that can catch and grasp insects. The physical environment of each island offered different food sources for exploitation as well as isolating its birds from other finch populations, preventing them from interbreeding and thus driving the evolution of new species.
Palaeoenvironmental reconstructions also provide the context for human morphological and behavioural evolution. Dietary preferences of the two-million-year-old hominin Australopithecus sediba from Malapa, South Africa were investigated using a combination of stable-isotope analysis, dental-microwear patterns and analysis of phytoliths extracted from dental calculus. Results show that Australopithecus sediba consumed a diet consisting mainly of tree leaves, fruits and bark, more similar to that of a chimpanzee than other contemporaneous australopiths in the region. Exploited food sources suggest that these hominins resided in a woodland environment.
Palynomorphs as a proxy for reconstructing palaeoenvironments
Fossil pollen and spores, acritarchs, and marine dinoflagellate cysts, chitonozoans and scolecodonts have much to tell us about past environments. Palynomorphs are present in sedimentary rocks from approximately two billion years ago to present, and in all sorts of environments. They represent parts of the life-cycles of plants and animals, which can be sensitive indicators of climate change, and as such palynology has great application for interpreting changing environments. In the Karoo Basin of South Africa, a climatic shift from glacial-type monosaccate pollen assemblages in the Late Carboniferous and Early Permian, to palynofloras dominated by taeniate bisaccate pollen in the Late Permian can be observed. As the climate became drier and more seasonal, taeniae on the main body of the pollen grain evolved as an adaptation to the swelling and contracting caused by considerable losses and gains of moisture in the environment.
Quantifying anthropogenic influence and the Sixth Mass Extinction
Ever-increasing human activity has transformed the planet, impacting on ecosystems, biodiversity, and species extinctions over the last 15 000 years. A key area of research is understanding when speciﬁc human activities, including hunting, land clearing and agriculture, began altering ecosystems at globally relevant scales. As such, the study of palaeoecology is very relevant to modern and future environments as well. Scientists have only been keeping detailed climatic records for a few decades, but much longer data sets are needed for accurate modelling in conservation ecology and projections of climate change. Palaeoecology can provide these critical data and improve our understanding of the Sixth Mass Extinction, which began with the disappearance of large mammals known as megafauna at the end of the last Ice Age, and continues today with plant and animal extinctions 100 – 10,000 times the natural rate. This is primarily caused by three factors: increased global concentration of greenhouse gases; oceanic devastation through overfishing and contamination; and the modification and destruction of huge areas of land and river systems to meet urban, industrial and agricultural needs.