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Continent Definitions Explained · Acedra
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0:00
you could say i've talked about continents fairly regularly on this channel and if you've really been paying attention i'd say you're familiar with the fact that people can't seem to agree on how many of these the earth actually has in school i was taught that there were one two three four five six seven though some of you might have been taught that there's a six or six and some people on youtube sure seem to like talking about an eighth but i think the big takeaway from all of this
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is that the term continent is rather vague and any way we decide to divvy up land will always be a little arbitrary but if that leaves you a little unsatisfied don't worry you're in good company which is why in search for a more definitive answer to what's ultimately an unanswerable question some people have turned to plate tectonics which is just kind of a mess what with all these little weird plates in between the big ones and so on others have done even worse and tried to
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use cultures in doing so giving themselves the impossible task of drawing straight lines through the murky divisions between groups of people and then there are some absolute fools who will say screw it let's just start from scratch and see where we get and yet here we are no closer to any sort of definitive answer and honestly maybe even a little bit more confused but okay what if we did something really crazy and say looked at the distribution of plants and animals across the world to see if
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there's any way to define continents through their native biology well it just so happens that my friends that is the beginning of
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biogeography now lucky for us someone has already gone through the trouble of dividing the world into seven biogeographic realms many of which line up nicely to our preconceived notions of continents unlucky for us in order to actually understand why these are the way that they are we can't just look at how things are today but rather we need to look much further back in time to see how past biology and past geography shaped our present biogeography or in other words we first need a lesson
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in paleobiogeography so buckle up while life has been found on this planet for over 3 billion years we only need to look back 200 million years to a time when all the continents of the world had come together into a single super continent pangaea this is an ideal time and place for us to start because having all major land masses physically connected allowed both plant and animal species to freely and completely disperse across any lands they found suitable briefly creating an
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original all-encompassing realm the pangea under which all terrestrial life was included while the biology of the pangea realm may have looked vastly different from what we're used to today it would be from this initial melting pot of a gene pool that all subsequent terrestrial life would delineate from the most recent common ancestor so to speak this era of interconnectedness came to an end as the tethos seaway began widening splitting the supercontinent into two
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roughly equal land masses larasia and gondwana sending the biota of each new continent off on their own unique evolutionary trajectory contained within larasia was north america europe and well most of asia while gondwana was a conglomerate of south america africa india australia oh yeah and antarctica from this point on the populations between the two continents effectively couldn't mix allowing for two distinct biogeographic realms to emerge the arctic and the tropic being centered
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at a much higher latitude laresia was periodically exposed to much lower average temperatures placing an evolutionary pressure on the plants and animals here to develop cold hardy bodies or else offered extinction to any species that couldn't keep itself warm a good example of how this pressure manifested itself would be in the pine genus or as the fancy scientists would call the penis genus having evolved on larasia some 130 million years ago pine trees quickly distributed themselves across the
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continent which is why even if we look at a modern distribution map for pine trees we'll see they still exhibit a luration distribution only growing in lands that once belonged to the arctic realm this environmental stress also affected animals making cold-blooded species less capable of surviving here while consequently giving warm-blooded animals like mammals the advantage this explains why it was here that the planet witnessed a huge diversification of mammals into the laresha theria a whole
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branch of mammals specific to lurasia which consists of many of the animals we're most familiar with today included in this development was perisodactyla or odd-toed undulates like horses donkeys tapers and rhinoceros artiodactilla or the even toad on gillette which holy crap includes even more popular animals like all camellids pigs ruminants like giraffes deers antelopes cattle sheep goats etc all the way to hippopotamus and even cetaceans rodintia which yeah are all rodents and
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carnivorous which can be further divided into feline and canine try to figure out what kinds of animals fit into those categories however just like its predecessor as laresia aged it too began to break apart with the central atlantic rift cutting a line between europe and greenland effectively splitting the arctic realm in two on one side the pale arctic the paleo in reference to this being considered by many to be part of the old world on the other side the new arctic or the new world arctic
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altogether the shared natural history of these two realms explains why basically all flora and fauna across north america has a direct analog to be found across eurasia and vice versa this similarity in biology was only reinforced with the periodic reconnection of asia to north america across the bering strait allowing further exchange between these seemingly distant lands okay so that's two out of the seven biogeographic realms meaning that the remaining five
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all came from the breakup of gondwana having stayed as a whole much closer to the warmth of the equator evolution on gondwana took its own path one that was less influenced by temperature resulting in a flora and fauna better adapted to tropical climates hence the title tropic for this realm and so just like how the laresia the area developed on eurasia here the gondwanath area began to emerge as well notably it was around 70 million years ago that the first true primates evolved to somewhere in
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the center of africa by this time gondwana had already begun splitting apart meaning primates weren't able to fully distribute themselves and instead were only able to reach south america and india this explains why the modern distribution of primates minus humans of course looks like this reflecting a late gondwanan dispersal as gondwana broke up even further so too did the tropical realm south america and africa became the largest fragments causing the gondwanatheria to diverge into the
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xenarthra and the afrotheria signifying the two continents becoming their own distinct biogeographical realms in the process the neotropic and the afrotropic from an assortment of common ancestors south america ended up with animals like the sloth anteaters and the armadillo while the afrotheria became things like aardvorks hydraxes and tinrics without any of the arctic megafauna this left room for larger herbivores to emerge from these groups so in south america common sloths were
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free to speciate into the enormous ground and aquatic sloths anteaters became well giant anteaters and the armadillo family diversified into glyptodonts in africa the same thing happened where a relative of the hyrax started growing bigger and bigger losing most of its hair while its nose got much longer giving birth to the order probasa dia the only surviving member of which being the elephant at one point a relative of the elephant even began living aquatically
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first staying within the shallows presumably using its trunk as a sort of snorkel but eventually fully adapted to the water and speciated into cyrenia becoming dugongs and manatees africa however never fully escaped the reach of lurasia having reconnected with asia approximately 30 million years ago this opened up the two lands for exchange with many of the larasian mammals coming down into africa filling the continent with even and odd towed ungulates felines and canines and so on
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relatives of the elephants also managed to make their way into the paley arctic adapting to the cold conditions by growing bigger and regrowing thick fur resulting in mammoths mastodons stegodantidae and moetherium by using the bering land bridge these species were even able to enter the knee arctic realm explaining how a genus originating in africa made it all the way to north america overall due to its more recent connection to eurasia the north of africa has largely been colonized by
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species of the pale arctic so much so in fact that it is now included as part of that realm as many of the species we associate most with africa lions rhinos zebras giraffes hyenas all got their evolutionary start in the arctic realm the afrotropic was still partially preserved however thanks to the sahara desert retaining its primates and most of its plant species and even produced us humans which as we all know went out from the afrotropic to extensively colonize
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every other realm on earth south america on the other hand remained isolated from about 80 million years ago until about 20 million years ago when the panamanian land bridge began to reach out from north america to join the two continents together this kickstarted what's known as the great american interchange a mixing of endemic species that continues to this day this wasn't a fair and equal fusion however as i already mentioned prior to this recombination north america has
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been sporadically reconnected to asia through the bering land bridge this not only brought the new arctic and the paley arctic realms temporarily back together but later this also meant african and indian influences reached all the way to north america the effect of this was that only the most competitive animals prevailed across these related realms stacking the knee arctic with far more robust species when compared to those hosted in neo-tropical isolation and so when these two realms collided
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many of the unique and specially adapted species of south america were replaced by the more generalist species of the north incoming undulates like camellids entered and evolved into llamas and alpacas felines came here to diversify into jaguars and cougars and even rodentia spread from north to south growing to sizes never before attained by rodents like capybaras and maras all the while helping to push native animals like a ground sloths and glyptodonts into extinction but this is
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also where the lines really start to blur as plants don't exactly follow the same rules as animals so while knee arctic animals were more successful overall at invading the neotropic the tropical climate of central america gave a localized edge to advancing neo-tropical plants the result of which being plant communities of south american origin coming to dominate all the way up to mexico where both the climate and therefore the flora begins transitioning to new arctic
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an interesting effect of this is that even though new arctic animals may be more widely adaptable in terms of animals especially adapted for tropical rainforests those of the neotropic still reign supreme allowing other neotropical animals to infiltrate deeper into north america and pushing the line of the neotropic as far north as the central american rainforests would reach overall this makes the whole of mesoamerica a tremendous transitional zone filling countries like mexico with
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some of the most diverse collections of flora and fauna to be found anywhere in the world a similar process of recombining realms can be found when looking at india you see india broke from gondwana approximately 130 million years ago and spent 80 million years adrift in the middle of what would become the indian ocean similar to south america this isolation bred a very unique biota and soon enough a distinct indian realm came about one derived from the greater tropical realm
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primates especially had a field day having come about only 70 million years ago their only route to colonizing india would have been a random rafting event meaning it was likely only a single primate species that found its way aboard this leaving ship but over the next 50 million years this common ancestor would come to diversify into a number of species what would become gibbons macaques and langers this isolation didn't last however and around 50 million years ago
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india came into contact with southern asia opening up a new arena for pale arctic and index species to interchange the only difference here is that while yes mixing did occur the arrival of india also led to the building of the himalayas which served as the world's biggest barrier for traversing plants and animals while these mountains shielded most of india preserving much of the indian realm an opening was left going through southeastern asia leading to a fusion of indian and malayan species
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the result being the modern day realm of indo-malaya this is all made slightly more complicated by the fact that only a few thousand years ago lower sea levels brought much more of the melee archipelago above the ocean forming the sunderland this opened an avenue for indo-malayan flora and fauna to expand as far as this shelf reached and remain here even after rising sea levels turned much of the region back into scattered islands why the indo-malayan realm doesn't
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include all of these islands however has to do with australia also once belonging to gondwana a joint australia and antarctica broke apart from the supercontinent at roughly the same time that india did the two southern continents remained attached in isolation for another 40 million years together forging a new australic realm rewind a bit and it was approximately 100 million years ago that the first marsupials evolved in modern-day north america from here they migrated south possibly
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using the caribbean islands to colonize south america while subsequently going extinct further north south america remained attached to antarctica by its southern tip until around 30 million years ago giving marsupials a pathway to expand further but it was also around this time that the southeast indian ridge started breaking the continents apart literally sending antarctica to the shadow realm while australia got lucky and pushed further northward right in the middle of this about 50
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million years ago when the two landmasses weren't too far removed yet a single marsupial managed to raft over to australia where it found a land devoid of most other mammals on account of the continent's isolation without any mammalian competition marsupials had finally found the one place where they could become the dominant animals of the realm this dominance allowed these single ancestral species to diversify into a variety of forms and functions with herbivores like kangaroos
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koalas and wombats and carnivores like quals tasmanian devils and thylacines the australic realm of course included a lot more different animals but hey it's easier to talk about specific examples than to mention every single living thing that found its way here but as time went on the australian landmass went through some changes losing zealandia while building up mass towards its front that would become new guinea as well as surrounding islands these islands would come to share in the
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biology of australia in the process forming the australasian realm but as these land masses pushed further and further north they eventually came into close proximity with the indonesian islands and by extension the indo-malayan realm so despite on a surface level appearing to be part of the same archipelago there are in fact two distinct groups of islands here separated by every biogeographer's favorite line the wallace line defining the border between indo-malaya
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and australasia of course as we should all know by now there are no straight or clear lines in nature and as a result of their proximity a new interchange has kick-started between the two realms as plants and animals crossed the narrow waterways between islands blending species from both realms together okay that's it right we're done oh yeah and then there was antarctica i'm pretty sure we all know how this one ends so i'll try to make this fast despite starting as the southernmost
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component of gondwana for the longest time antarctica was a highly habitable land and enjoyed a golden age brought about by the global greenhouse climate of the earth at the time during this period it would have closely resembled neighboring australia which stayed connected until about 100 million years ago finding its spot at the south pole by around 80 million years ago even at the bottom of the planet antarctica remained warm enough for life to thrive for another 30 million years or so
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in this time antarctica's biology likely diverged substantially from gondwanan and even australian biology to the point where a new biogeographic realm was realized the antarctic realm but as we all know the planet began to cool as it entered a new ice age just 3 million years ago and nowhere on earth was affected more by this change than antarctica year after year temperatures steadily dropped soon enough ice began to build up and didn't stop building until the whole continent was
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under literal miles of ice extinguishing virtually all native antarctic life in the process putting the wide scale extinction aside though the true shame of this is that all the fossils left behind now lay buried under miles of ice meaning whatever unique biology that could have developed here before the freeze remains utterly hidden and unknowable to us now leaving antarctica's paleobiology as a huge blind spot within the field altogether this leaves the earth with one two three
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four five six seven distinct terrestrial realms well actually i didn't want to bring this up in the beginning but there's actually a little bit of an eighth realm and no it's not zealandia but it's actually not that far off either you see if you watched my earth's lost land masses video you'll know that the pacific ocean is the site of a lot of new land building processes both as a result of tectonic uplifts and continuous volcanism constructing islands entirely independent of
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all other lands on earth with no natural history to draw from each newly created island must build their own distinct flora and faunas from whatever seeds birds or the tides may bring over being situated within the pacific ocean which covers a third of the planet's surface means the islands of the pacific are perfectly situated to receive inputs from nearly all of the biogeographic realms save for the afrotropic what's been created is a fascinating laboratory where
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all sorts of different combinations between realms can take place producing a unique array of biology on every single island if this wide range of potential inputs wasn't enough once these castaway species reached their final island destinations from here they set off on their very own journey of evolutionary exploration one that's defined by whatever peculiar environmental conditions are harbored therein in turn it is these island dwellers that are most active in seeding whatever new
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islands might arise within the pacific volcanic arena what this all means is that while this prismatic gene pool makes it impossible to generalize the oceanic realm it nonetheless constitutes in and of itself a distinct biota connected through a disconnection to any others universal in its uncategorized ability hawaii is a fascinating case study of all of these principles at work not only does the hawaiian island chain represent the largest islands within the realm but also some of the most
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isolated being 3 700 kilometers from the nearest continental lands of california part of the new york deck 5300 kilometers from pale arctic lands 8 500 kilometers from the indomalangan realm 6400 kilometers from australasia and 8400 kilometers from the neotropic despite being located closest to north america prevailing ocean currents lead the waters of the northern pacific to circulate counter-clockwise acting as a conveyor belt delivering material from all over the ocean to these islands
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this explains why when analyzing the plant species present and deciphering their realm of origin it was found that indo-malaya was the greatest source of native hawaiian plant species having supplied 21 percent of hawaiian flora despite being the furthest away the new arctic was the second most influential that's unsurprising supplying 19 percent what's really interesting however is just how much was contributed by other areas east asia was next contributing 17
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of native hawaiian species the neotropic supplied 12 and even 9 percent originated from far away australasia if you're following along and doing the math in your head you'll find that there's still about 22 percent of the hawaiian gene pool that's unaccounted for and this is truly where the eighth biogeographic realm comes into play as all of the islands of the pacific also receive influx from all of the other islands of the pacific and it's estimated that an additional 11
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percent of the hawaiian gene pool comes from other pacific islands this leaves a final 11 of native hawaiian species with unknown origins because while figuring out the natural history of things like this can be extremely tricky but consider this in the last 400 000 years a new island has begun forming off the southeastern edge of hawaii forming the loewy high seamount while luihai remains 975 meters under water today with an average growth rate of 5 meters per millennia
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at some point over the next 200 000 years a new island will breach above the ocean once this occurs sure louie high will receive inputs from all around the pacific but the most immediate donor will of course be the other hawaiian islands what this appears to suggest is a chain of donation from one island to the next in perpetuity and looking at how far back the hawaiian islands stretch we begin to see the potential for a lineage passed down from island to island to island all
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the way to the present it's possible that this inherited hawaiian biota may in fact be the source of the remaining unidentified 11 percent of species native to the islands altogether what i hope this has shown is that the islands of the pacific share an ineffable quality that make them unlike anywhere else on earth uniting them into a single patchwork realm oceania and so there you have it that's the seven or eight realms of biogeography so what do you think is this a better
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way to divide and talk about our world compared to the six or seven or eight continents or are you still unconvinced well either way let me know by leaving a comment and if you enjoyed this trip into the world of biogeography and would like to see me dive deeper into the field then don't forget to subscribe so you don't miss any more videos i put out and of course if you want to help support me in continuing to make these videos you can head over to my patreon lastly if you're craving more
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biogeography i'd suggest you check out this video i made recently on the ice age thanks
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