If you cast your gaze back 66 million years, a fleeting moment in Earth’s history, you might have beheld a brilliant flash of light as a colossal asteroid, akin to a mountain in size, hurtled through the atmosphere and collided with our planet. This cataclysmic event, transpiring in the springtime of that age, marked the literal culmination of an epoch—the Mesozoic era.
Should you have miraculously endured the initial impact, you would have borne witness to the havoc that ensued. Enormous firestorms, colossal megatsunamis, and an enduring nuclear winter, spanning months to years, cast an ominous shadow over the world. In the blink of an eye, the 180-million-year dominion of non-avian dinosaurs met its tragic end, as did at least 75% of the myriad species that had once shared the Earth’s embrace.
This pivotal juncture, known as the Cretaceous-Paleogene mass extinction (K-Pg), ushered in a new era for our planet. Ecosystems, though profoundly altered, eventually rebounded, yet the life forms that inhabited them bore little resemblance to their antecedents.
Numerous iconic pre-K-Pg species now reside solely in the confines of museums. The formidable Tyrannosaurus rex, the nimble Velociraptor, and the winged denizens of the Quetzalcoatlus genus all succumbed to the asteroid’s cataclysmic embrace, relegated to the annals of deep history. However, should you step outdoors and inhale the fragrant aroma of roses, you stand in the presence of ancient lineages that flourished amidst the ashes of the K-Pg catastrophe.
While the contemporary species of roses differ from those that coexisted with Tyrannosaurus rex, their lineage, belonging to the family Rosaceae, originated tens of millions of years prior to the asteroid’s fateful arrival. Roses, in this respect, are not an anomaly among angiosperms (flowering plants). Fossil records and genetic analyses suggest that the majority of angiosperm families emerged prior to the asteroid’s impact.
It appears that angiosperms, like the early progenitors of our own lineage—the mammals—seized the opportunity presented by this fresh start. However, the mechanism by which they accomplished this remains enigmatic. Unlike the formidable dinosaurs, angiosperms are delicate, incapable of flight or rapid mobility to evade harsh conditions. Their very existence depends on sunlight, which was temporarily obscured.
Our comprehension of this event is obscured by the varied accounts offered by fossils unearthed from different regions. It is evident that there was a substantial turnover of angiosperm species in the Amazon when the asteroid struck, accompanied by a decline in herbivorous insects in North America, suggesting a depletion of their food plants. Yet, in some regions, such as Patagonia, no discernible pattern emerges.
In a 2015 study scrutinizing angiosperm fossils from 257 genera (each family typically encompasses multiple genera), it was suggested that K-Pg had a negligible impact on extinction rates. However, extrapolating this result to encompass the 13,000 angiosperm genera proved challenging.
In collaboration with my colleague Santiago Ramírez-Barahona from the Universidad Nacional Autónoma de México, we adopted a novel approach to resolve this conundrum. Our study, published in Biology Letters, involved the analysis of extensive angiosperm family trees, constructed from DNA sequence mutations spanning 33,000 to 73,000 species. This approach, rooted in the practice of constructing family trees, has historically paved the way for profound insights into the evolution of life, ever since Charles Darwin’s pioneering work.
Although these family trees did not encompass extinct species, their structure provided vital clues regarding shifts in extinction rates across epochs, as evident through the ebb and flow of branching rates. Mathematical models were employed to estimate the extinction rate of angiosperm lineages, juxtaposing ancestor age with the projected number of species according to our understanding of evolutionary processes. This analysis also considered the pace at which new species evolved, yielding a net diversification rate—essentially, how swiftly new species arose, adjusted for the species that vanished from the lineage.
The model generated temporal bands, such as spans of a million years, illustrating the fluctuations in extinction rates over time. Moreover, it enabled the identification of periods marked by heightened extinction rates. This approach also gauged the extent to which DNA evidence corroborated these findings.
Our investigation unveiled a striking revelation: extinction rates within angiosperms remained remarkably consistent over the past 140 to 240 million years. This remarkable stability underscores the resilience that angiosperms have exhibited across eons.
It is important to acknowledge the fossil evidence attesting to the loss of numerous angiosperm species during the K-Pg event, with certain locales suffering more severe consequences than others. Nevertheless, our study appears to confirm that the lineages, both families and orders, to which these species belonged persevered undisturbed, ultimately shaping the world as we know it today. This differs markedly from the fate of non-avian dinosaurs, who were entirely extinguished, their branch severed from the tree of life.
Scientists posit that the tenacity of angiosperms during the K-Pg mass extinction may be attributed to their adaptability. They evolved novel mechanisms for seed dispersal and pollination, adapting to their changing environment. Additionally, their ability to duplicate their entire genome provides a redundancy of genetic material on which natural selection can operate, potentially yielding novel forms and greater diversity.
The current juncture marks the onset of the sixth mass extinction event, and a disconcerting number of angiosperm species already teeter on the brink of extinction. Their demise could usher in a transformative change, reshaping life on Earth as we recognize it. Indeed, angiosperms may once again flourish from a diverse cohort of survivors, potentially outlasting our own species.
Source: The Conversation