The Continuing Importance of Paleontology
As my-coauthors and I argue in a recently published opinion piece in Scientific American, paleontology is far more than new fossil discoveries. As exciting as the latest dinosaur or Burgess shale fossil may be, their true importance is how they contribute to our understanding of the 3.5 billion history of life on Earth and how this history records life’s responses to the challenges caused by environmental changes. Our understanding of this history gives us an invaluable perspective on the challenges caused by current and future environmental vicissitudes. Paleontology demonstrates that adequately understanding the modern biosphere depends on knowing its ancient past; biology cannot be ahistorical. Three recent papers clearly demonstrate this.
It has long been known that there are more species of insect in the world than any other group of animals. The most diverse group of these, with immense ecological importance, are the herbivorous insects, those that feed on plants. A long-standing question about this group is: what factors are responsible for this immense diversity? One option is that it is a direct reflection of the number of host plants: the more diverse the plants, the more diverse the herbivorous insects. Another, not mutually exclusive option, is that it reflects how many types of insect, each feeding differently, can feed on a single type of plant, what is termed “niche packing.” In a paper in the Proceedings of the National Academy of Sciences (aka PNAS), Jorg Albrecht and colleagues apply paleontological data to this problem. They take advantage of an approach developed and refined over many years: the analysis of leaf damage types. The mouthparts of insects and their mechanism of feeding leave distinct feeding marks (damage) on leaves, which can be identified in fossils. The more damage types on the leaves of a particular group of fossil plants, the more types of insects are responsible for them. Using data on damage types from thirty-two fossil floras, plus sophisticated statistical analyses, the authors were able to show that niche packing was the principal driver of an increase in herbivorous plant diversity over the last 66 million years (the Cenozoic). In their words, this study “highlights how the fossil record can be used to test fundamental theories of biodiversity.”
Until about 13,000 years ago, North America was populated by a diverse assemblage of large mammals (megafauna), such as mammoths, mastodons, ground sloths, short-based bears, and dire wolves (not the Game of Thrones kind). The cause of the extinction of these animals, in what is called the Late Quaternary Megafauna Extinction, has long been a subject of contention. Many have argued for climate change at the termination of last ice age as the main culprit, while others have contended that it was the activities of humans, especially hunting, that wiped out the megafauna. Others have suggested both played a role. In any case, the loss of so many herbivores and carnivores has had ecological impacts that reverberate to this day. In a study published in Science, O’Keefe and colleagues leverage the huge number of mammals collected from Rancho La Brea in Los Angeles (“the LaBrea tar pits”) to address this fraught topic. Based on carbon dating, they were able to accurately determine the timing of the extinction event. They compared this with the climate and fire history recorded in nearby oceans and a lake, as well as the record of humans in the region. They found a record of gradual drying and a rapid transition from woodland to chapparal ecosystems. Remarkably, there was a period of intense fire activity coincident with the shift between the two ecosystem types, the extinction episode, and the appearance of humans. They propose that human set fires, spread by drying vegetation, promoted the ecosystem shift. This shift, coupled with human hunting, could have led to the extinction. As the authors say in a separate essay: “Our research suggests that the combination of heat, drought, herbivore loss and human-set fires had pushed this system to a tipping point….Studying the causes and consequences of the Pleistocene extinctions in California can provide valuable context for understanding today’s climate and biodiversity crises. A similar combination of climate warming, expanding human populations, biodiversity loss and human-ignited fires that characterized the ice age extinction interval in Southern California are playing out again today.”
A key question on assessing the impact of humans on ancient ecosystems in the Americas is, when did humans first arrive? For a long time, the answer was somewhere between 16,000 to 13,000 years ago, coincident with the end of the ice age (and the megafaunal extinction). In 2021, researchers at White Sands National Park, New Mexico, announced that they had discovered ancient human footprints preserved in sediments along the margin of a fossil lake. Based on seeds preserved in the same layers as the footprints, humans walked in the area between 23,000 and 21,000 years ago, during the last glacial maximum, millennia before they were thought to be in North America. As is the nature of science, their dating results were vigorously challenged. In a follow-up paper just published in Science, Pigati and co-workers report on independent verifications of the age given in the earlier paper, using different methods, including fossil pollen. Equally interesting to the confirmed age is the presence of the tracks of megaherbivores (mammoths and ground sloths) along with the human tracks. As is also the case for good science, the results raise even more questions: Why is there a gap in the record of humans in the Americas? What was the interaction between the megafauna and humans? As the authors put it, “The overlap of humans and megafauna for at least two millennia during this time suggests that if people were hunting megafauna the practices were sustainable, at least initially.” Although this paper, since it involves humans, may seem the province of archaeology, the study of footprints (ichnology), fossil seeds and pollen (paleobotany and palynology) and of the megafauna make it also paleontological.
As we pointed out in our essay, paleontology established the reality of extinction and thus of a dynamic diversity of life. As these papers, and so many others demonstrate, paleontology also documents the ever-changing nature of ecosystems. Recognizing that historical context is critical to understanding modern processes and patterns and their susceptibility to future changes.
References:
Albrecht, J., T. Wappler, S. A. Fritz, and M. Schleuning. 2023. Fossil leaves reveal drivers of herbivore functional diversity during the Cenozoic. Proceedings of the National Academy of Sciences 120(32):e2300514120.
O’Keefe, F. R., R. E. Dunn, E. M. Weitzel, M. R. Waters, L. N. Martinez, W. J. Binder, J. R. Southon, J. E. Cohen, J. A. Meachen, L. R. G. DeSantis, M. E. Kirby, E. Ghezzo, J. B. Coltrain, B. T. Fuller, A. B. Farrell, G. T. Takeuchi, G. MacDonald, E. B. Davis, and E. L. Lindsey. 2023. Pre–Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science 381(6659):eabo3594.
Pigati, J. S., K. B. Springer, J. S. Honke, D. Wahl, M. R. Champagne, S. R. H. Zimmerman, H. J. Gray, V. L. Santucci, D. Odess, D. Bustos, and M. R. Bennett. 2023. Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science 382(6666):73–75.
