Today's marine mammals have increased tremendously in size since the Cambrian period ended 485 million years ago and scientists now know why.
Scientists already agree these animals have gotten bigger, on average, and they say it's not by random evolution, according to a new study. They conclude this super sizing could be because bigger animals have better chances of growing larger at sea.
The study noted that over the past 542 million years, the average size of a marine animal has jumped by a factor of 150. The Blue Whale, today's largest mammal, can reach lengths of over 100 feet, which is more than 100,000 times the size of the largest animal in Cambrian period when all life existed in the oceans
Dr. Noel Heim from Stanford University in California did a detailed analysis based on a huge trove of scientific studies about prehistoric life and came to conclusions that revealed the trend towards increasing size.
His team also wanted to determine if this trend towards bigness was really caused by evolutionary advantage or was simply a matter of random chance.
Of the many computer simulations they developed, the best that matched the real fossil history was one with a genuine size advantage.
"The degree of increase in both mean and maximum body size just aren't well explained by neutral drift," said Dr. Heim to Sci-News. "It appears that you actually need some active evolutionary process that promotes larger sizes."
He said growing larger helped an animal move faster or eat larger prey. He also noted an increase in oxygen in the oceans might also have played a role in this dramatic increase in animal body sizes.
Researchers found that when land animals like reptiles and mammals returned to the water, the body sizes of these animals showed a tremendous increase.
More tellingly, it was the return of mammals that eventually produced today's massive whales, the largest mammals on Earth.
"You can see in the data, there is this trend to increasing size -- but a real kick comes when you have these air-breathers re-entering the water," said Dr. Michael Berenbrink, an evolutionary physiologist at the University of Liverpool, to the BBC.
"Then you have a sort of a step change in size."
He said breathing air delivers much more oxygen to an animal's tissues, allowing them to sustain bigger bodies underneath the waves.