Go big or go home
Image Credit: lightpoet via Envato
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In 2012, Argentinian shepherd Aurelio Hernández set off for work one fine day to search for his flock on the ranch he worked in Patagonia. But instead of sheep, he came home with reports of huge bones sticking out of the dusty, dry landscape. Subsequent excavation revealed a dinosaur graveyard, with at least six huge sauropod dinosaurs becoming stuck in the mud on what, around 100 million years earlier in the late Cretaceous, would have been a swampy floodplain.
One of the specimens, a new species of titanosaur named Patagotitan mayorum, caused particular excitement. Although its exact size is up for debate, with its thighbone standing 2.38 metres (or almost 8 feet) tall, this individual would have clocked in at something like 40 metres from nose to the tail with a body mass just shy of 70 tonnes - measurements that would make this animal the largest terrestrial species on record.
Giants have a knack of capturing the imagination. Whether it’s a new titanosaur, tales of mythical giants, towering redwoods or the awe inspired by a close encounter with a whale, everyone loves REALLY BIG THINGS.
Go big or go home is a philosophy that goes way back in time: from the moment that multicellular organisms became a thing around 3-billion years ago, the stage was set for the emergence of absolutely massive organisms. Gigantism is a recurring leitmotif in evolution’s playlist. There are giants – now extinct or occasionally still with us – in just about every multicellular branch of the tree of life.
In the coral reefs of the South Pacific and Indian oceans, there are giant clams that are more than a metre long and can weigh over 200 kg. There’s an ammonite - a type of snail-like mollusc - from the late Cretaceous whose 3m diameter would have made it too large for the mantlepiece. Uncoiled, its spiralling body would have stretched to almost 20 metres, comparable in length to the largest specimens of the extinct giant shark Otodus megalodon and the still-living giant squid.
On land, there have been – and still are – giant insects. Open your hand wide and you’ll just about cover the length of the largest living beetle Titanus giganteus, an impressive 17cm, or more than six and a half inches.
And if that makes you shudder, you probably won’t want to contemplate the biggest insect on record, a colossal dragonfly – sometimes called a griffinfly – from the Permian era almost 300 million years ago, with a body length of almost half a metre and a colossal 71 centimetre wingspan.
Big birds aren’t just for Sesame Street. Given that they’re descended from dinosaurs, known for their size, it’s unsurprising that there are some impressive feathered giants flapping through the fossil record. One was a vast pelican-like bird called Pelagornis, whose 5 metre-plus wingspan would have cast a shadow across the entire length of a Cadillac car.
Reconstructions of Phorusrhacids, an extinct species of flightless predatory bird that roamed earth up to around 2 million years ago, are genuinely scary. Often referred to as “terror birds”, the largest individuals would have towered over the average adult human. They had the thunking great thigh-bones of a sprinter, fiercely sharp talons and an eagle-like beak that would have ripped into their much smaller mammalian prey. Not so much “Who’s a pretty boy?” and more “Oh God, please don’t eat me!”
In time, however, the mammals had their turn at being giants too. There are crowd-pleasers like Megatherium, the giant ground sloth that, standing on its hind legs, could have chewed on leaves some 6m above the ground. Then there’s the Irish elk Megaloceros with antlers that spanned more than 3.5 m from point to point, and the armadillo-like Glyptodon which was roughly the size of a Volkswagen Beetle. And who can resist a giant beaver? Castoroides, sadly no longer with us, was probably a hefty 100 kilos and had front teeth as long as your face.
Almost all of the world’s known megafauna are now extinct - some vanishing relatively recently in evolutionary terms, in the past 40,000 years or so. What’s left is the African elephant as the largest living land mammal and the blue whale - with a fluke-to-snout distance of over 30m and a body mass of over 150 tonnes, the largest living marine mammal. In fact, the blue whale is the largest animal on record. Period.
There are hundreds of other examples beyond this list, but what’s clear from this quick survey of the tree of life is that giant animals have appeared over and over again in the history of life on earth. And what this repeated independent evolution of gigantism strongly suggests is that being not just big but really, REALLY big must have its uses.
Within the animal kingdom, one obvious benefit of being massive is that you have more muscle power. While it takes a lot of food to keep you alive, being big allows you to commandeer more resources, marginalising or metaphorically trampling over more diminutive species that might be after the same stuff
The other significant upside of being bigger than everything else in your ecosystem is that you reduce the risk of being consumed by something else. Both these selective forces were almost certainly in play during the evolution of nature’s giants, says Geerat Vermeij, a geologist at the University of California at Davis, who has studied the conditions that may have been needed for gigantism to evolve. “You can’t become gigantic without at least some degree of food supply, but it’s probably things like competition and predation that compel lineages to become very large,” he says.
When it comes to understanding the genetics of nature’s giants, we don’t actually know all that much, but the genome of the world’s largest living rodent – the capybara – does give us some pointers. It’s no blue whale or African elephant, but this famously chilled-out semi-aquatic mammal is a veritable behemoth compared to its ratty relatives. With an average body mass of 55 kilos, the capybara is around 2000 times bigger than a mouse and 60 times larger than its closest living relative, the guinea-pig-like rocky cavy.
The capybara evolved from a 1 kilo ancestor in the space of just 20 million years or so - that’s incredibly fast in evolutionary terms, making the capybara an interesting muse for those intent on understanding the genetics of gigantism. And when evolutionary biologist Santiago Herrera Alvarez and his colleagues scoured its genome, they found evidence of accelerated evolution in 12 genes and gene families involved with cell proliferation in early development and bone growth after birth.
Being massive, for all its advantages, does have some significant drawbacks. One is that building and maintaining a big animal requires a heck of a lot of cells. And that means a heck of a lot of cell division, which should – in theory – increase the risk of rogue cells popping up that could lead to cancer.
Curiously, this hypothesis doesn’t seem to hold up - a fact first noted by epidemiologist Richard Peto in the 1970s, who found that bigger, longer-lived animals do not appear to have a higher prevalence of cancer. If anything, it’s the other way round, with large, long-lived animals like elephants, whales and even our friend the capybara having unusually low rates of cancer compared with smaller creatures.
So, what’s going on? Well, the solution to the eponymous Peto’s paradox, like so much in biology, lies in evolution.
If the incidence of cancer begins to creep up in a species, it will be those individuals with a chance cancer-proofing mutation that will be more likely to survive to reproductive age and pass on their genetic innovations to the next generation. Support for this evolutionary explanation is to be found in a survey of the best-studied cancer protective tumor suppressor gene, TP53, across a range of animals of various sizes.
Many mammals, including humans, have just one TP53 gene. In the human genome, our TP53 gene is found on the short arm of chromosome 17. We all have two copies, or alleles - one each from mum and dad - and if one of these copies is defective, this results in Li-Fraumeni Syndrome - a hereditary condition that leads to around half of those with it developing one or more cancers by the age of 30.
So it makes sense that evolving extra backups of TP53 would provide more protection, acting like an insurance policy against cancer. It is intriguing then to learn that as the elephant family tree branched to generate new lineages of increasingly massive mastodons and mammoths, there appear to have been duplications of the TP53 gene.
The cells of modern elephants boast not one but 20 versions of TP53, adding up to extraordinarily efficient protective DNA-damage responses, which may help account for the very low levels of cancer in these giants. As soon as a cell gets damaged, it dies, so there’s no opportunity for it to turn into a potentially lethal tumour.
While extra copies of TP53 might be the solution to Peto’s paradox for elephants, it’s not the only genetic trick in town. For the capybara, natural selection appears to have settled for enhancements in three other gene families, one involved in tumour reversion (where cancer cells switch back from their bad behaviour to become more normal), a second that flags up potentially cancerous tissue to the immune system and a third that plays a role in programmed cell death, or apoptosis, which is a way of getting rid of damaged cells before they can cause trouble.
Aside from cancer, being gigantic poses another significant challenge. Big animals have large appetites so there’s a limit how many giants any given ecosystem can sustain. And small populations of large, slow-to-reproduce individuals are not particularly stable, so they tend to be the first species to go in a mass extinction. This may help explain giants – every time they have evolved – do not last long. As Geerat Vermeij puts it, “Large size has never been a long-term advantage.”
But if there’s one thing that the fossil record tells us, it’s that life always finds a way and, in time, new lineages of gigantic creatures will emerge. They’ll be back.
