Sharks have roamed the Earth for at least 450 million years, and to this day, they continue to reign supreme in the vast oceans.
Sharks aren’t the largest creatures in the ocean, nor are they always at the top of the food chain. They’ve had numerous adversaries throughout their evolution.
Yet, they’ve persisted for over 450 million years, surviving four out of five mass extinction events, including the cataclysm that wiped out the dinosaurs 66 million years ago at the end of the Cretaceous period.
Sharks are even considered “older” than dinosaurs, which only appeared around 240 million years ago, and they predate terrestrial vegetation, which emerged about 390 million years ago.
What has enabled sharks to endure for such an astonishing length of time remains a mystery that continues to baffle scientists.
How Have Sharks Survived?
One widely accepted theory is that sharks possess the unique ability to alter their physiological mechanisms to adapt to changing environmental conditions. For instance, they might shrink in size when temperatures rise and do the opposite when temperatures fall.
This adaptability allows sharks to quickly adjust to different ecological niches, which are constantly shifting within the ocean depths.
A distinguishing characteristic of sharks, as well as some of their close relatives like rays and chimeras, is that most of their skeletons are made of cartilage instead of bone.
This special structure allows them to modify their size more easily, enhancing their ability to adapt to various environments.
Research, for example, has studied the ocellated skate (*Leucoraja ocellata*) in the southern Gulf of St. Lawrence, Canada. This species has adapted to a 10°C increase in water temperature over 7,000 years by reducing its size by up to 45%.
In evolutionary terms, 7,000 years is a very short time frame, and this rapid size change suggests that environmental factors, rather than gradual natural selection, could have driven this adaptation.
Additionally, sharks have the advantage of a vast genome accumulated over millions of years of evolution. Scientists believe this genome may contain genetic codes that, while seemingly unnecessary today, helped sharks survive harsh climatic conditions in the past.
Some shark species have also adapted to move between freshwater and saltwater environments—a significant physiological challenge. This ability may have been crucial in the past when global temperatures fluctuated, causing massive amounts of freshwater to flow into the oceans as ice sheets melted.
The Greatest Threat: Humanity
The role of sharks in maintaining the balance of marine ecosystems cannot be overstated. Their survival has been integral to keeping the oceans rich and diverse.
However, sharks now face an unprecedented threat: human activity and its transformative impact on global ecosystems.
Gavin Naylor, director of the Florida Program for Shark Research, stated, “Sharks may have dealt with climate changes well in the past, but the greatest challenge sharks and rays face today is overfishing.
“These creatures cannot outcompete humans—they can’t escape being pulled out of the water,” Naylor emphasized.
Moreover, the impacts of ocean pollution and habitat loss are additional challenges sharks currently face.
“If these animals are threatened, the oceans will be disrupted, potentially leading to a cascade of changes in marine ecosystems that Earth has never seen before,” Naylor warned.
