Several times I’ve mentioned some similarities between people and Bunny Rabbits and suggested that we must have a common ancestor not too far back. Further than the primate divide, but less than the deer / horse / bunny division.
Well, I finally got to look for it. The clade (group) is called “Glires”.
Rabbits and people can both eat onions. For many other herbivores, they are toxic.
Rabbits have a caecum that acts as a fermentation pouch. Like the stomachs of cows and sheep, but after the stomach and small intestine instead of in front. We have an appendix that is a ‘no longer used’ version of the same kind of intestinal fermentation pouch. (Some folks have postulated that the appendix may actually make some vitamins for us and might not be completely unused, thus the quotes).
There are other similarities, but those are two that stand out.
For a general and non-technical description:
For a more technical version:
The rabbit’s caecum is the largest organ in the digestive tract and it develops over about 1 month from birth to be truly functional. Initially young rabbits depend for their nutrition on milk which is high in fat and protein but contains only small amounts of lactose. The C8 and C10 fatty acids in milk fat are bacteriostatic and protect the young animal from invasion by detrimental organisms. As milk intake diminishes towards weaning the organisms that develop in the gastrointestinal tract particularly in the caecum depend on the type of carbohydrate and the protein flow into the caecum. Recent research has demonstrated an adverse effect of high protein diets on survival of rabbits after weaning and throughout the fattening period.
The rabbit has the ability, through specialized musculature of the large intestine, to direct slowly fermented fibrous feed into the colon for excretion in the hard faeces. The same mechanism separates small particles (more digestible fiber) and soluble components of digesta leaving the ileum into the caecum where it remains in a buffered medium supporting bacterial growth. The rabbit quickly “sieves” indigestible or slowly fermentable fibers which would slow its feed intake were they to enter the caecum. Hard faeces are produced some 4 hours after a meal.
Later the bunny will excrete a fermented pellet mass and eat it. This second pass lets them get a lot more nutrients from plant food along with a variety of vitamins as fermentation products. This looks like they are eating their own poo, but it’s a very important process. It also makes them non-halal, something for which rabbits are undoubtedly very grateful ;-)
So I’d wondered just how far back up stream we had to go to find a common ancestor / branching point, and who else might be in this clad with humans, primates, and rabbits (lagomorphs)? As the labomorphs were only recently divided from the rodents, I was pretty sure we’d be lumped in with rats and mice, too; but look at most politicians and that’s pretty clear ;-) The division is due to the unique teeth of bunnies. Rodents have 4 front teeth, bunnies have six. The top set has two in line, front to rear, in each of the usual two incisor locations. The lower incisors are inserted between the two upper set and make a very nice snipping device.
But who else?
This clade joins the rodents and the bunnies. We join them just one more step up.
Glires (Latin glīrēs, dormice) is a clade (sometimes ranked as grandorder) consisting of rodents and lagomorphs (rabbits, hares, and pikas). This hypothesis that these form a monophyletic group has been long debated based on morphological evidence, although recent morphological studies strongly support monophyly of Glires (Meng and Wyss, 2001; Meng et al., 2003). In particular, the discovery of new fossil material of basal members of Glires, particularly the genera Mimotona, Gomphos, Heomys, Matutinia, Rhombomylus, and Sinomylus, has helped to bridge the gap between more typical rodents and lagomorphs (Meng et al., 2003; Asher et al., 2005). Data based on nuclear DNA support Glires as a sister of Euarchonta to form Euarchontoglires (Murphy et al. and Madsen et al. 2001), but some genetic data from both nuclear and mitochondrial DNA have been less supportive (Arnason et al. 2002). A study investigating retrotransposon presence/absence data unambiguously supports the Glires hypothesis (Kriegs et al. 2007).
So what is one step up? I mention glires as the next step up is Euarchontoglires or glires with a modifier in front…
Euarchontoglires (synonymous with Supraprimates) is a clade of mammals, the living members of which are rodents, lagomorphs, treeshrews, colugos and primates (including humans).
Yes, we and the other primates are more closely related to ‘treeshrews’ and ‘colugos’ (whatever they are…) than to bunnies and rodents, but that’s not really all that big a divide. Far far away are things like cats, dogs, horses, elephants, etc. etc.
The Euarchontoglires clade is based on DNA sequence analyses and retrotransposon presence/absence data, combining the Glires clade, which consists of Rodentia and Lagomorpha, with that of Euarchonta, a clade consisting of Scandentia, Primates (which includes humans) and Dermoptera.
Euarchontoglires is now recognized as one of four major groups within Eutheria (containing placental mammals). These four clades are usually discussed without a Linnaean rank, but has been assigned the rank of cohort or magnorder, and superorder. Relations within the four cohorts, Euarchontoglires, Xenarthra, Laurasiatheria, and Afrotheria, and the identity of the placental root, remain somewhat controversial.
Euarchontoglires probably split from the Laurasiatheria sister group about 85 to 95 million years ago during the Cretaceous, developing in the Laurasian island group which would later become Europe. This hypothesis is supported by fossil as well as molecular evidence. The clade of Euarchontoglires and Laurasiatheria is recognized as Boreoeutheria.
Have I mentioned lately that the shift from Linnaean naming to cladistics was underway when I was in college, and it is the reason that I decided not to go into anything that required learning cladistic naming conventions? I was not going to learn one whole set of naming system just to throw it all out and learn a new one… ;-)
At any rate, what are these things?
Scandentia are the treeshrews.
The treeshrews (or tree shrews or banxrings) are small mammals native to the tropical forests of Southeast Asia. They make up the families Tupaiidae, the treeshrews, and Ptilocercidae, the pen-tailed treeshrews, and the entire order Scandentia. There are 20 species in 5 genera. Treeshrews have a higher brain to body mass ratio than any mammals, including humans, though this is not uncommon for animals weighing less than a kilogram.
Although called treeshrews, they are not true shrews (although they were previously classified in the Insectivora), and not all species are necessarily arboreal. Among other things, they eat Rafflesia fruit.
Among orders of mammals, treeshrews are closely related to primates, and have been used as an alternative to primates in experimental studies of myopia, psychosocial stress and hepatitis.
So our close relatives like eating bugs, too…
The demoptera or colugos are an interesting group. Flying lemurs:
Colugos ( /kəˈluːɡoʊ/) are arboreal gliding mammals found in South-east Asia. There are just two extant species, which make up the entire family Cynocephalidae ( /ˌsaɪnəsɛˈfɑːlɨdiː/) and order Dermoptera. They are the most capable of all gliding mammals, using flaps of extra skin between their legs to glide from higher to lower locations. They are also known as cobegos or flying lemurs, though they are not true lemurs.
Colugos are fairly large for a tree-dwelling mammal: at about 35 to 40 centimetres (14 to 16 in) in length and 1 to 2 kilograms (2.2 to 4.4 lb) in weight, they are comparable to a medium-sized possum or a very large squirrel. They have moderately long, slender limbs of equal length front and rear, a medium-length tail, and a relatively light build. The head is small, with large, front-focused eyes for excellent binocular vision, and small, rounded ears. When born, a colugo weighs only about 35 g (1.2 oz) and does not reach adult size for 2–3 years.
Their most distinctive feature is the membrane of skin that extends between their limbs and gives them the ability to glide long distances between trees. Of all the gliding mammals, the colugos have the most extensive adaptation to flight. Their gliding membrane, or patagium, is as large as is geometrically possible: it runs from the shoulder blades to the fore-paw, from the tip of the rear-most finger to the tip of the toes, and from the hind legs to the tip of the tail; unlike in other known gliding mammals, even the spaces between the fingers and toes are webbed to increase the total surface area, as in the wings of bats. As a result, colugos were traditionally considered being close to the ancestors of bats, but are now seen by some as the closest living relatives to primates.
So I guess my desire to fly and leap out of airplanes and trees is natural ;-)
Quite an interesting collection of close relatives. Rodents, bunnies, treeshrews, flying lemurs, and primates of various sorts.
So now you know.
Rodents includes not just rats and mice, but squirrels, guinea pigs, and hamsters along with beavers and porcupines. So when someone says we’re being ‘squirrelly’ they might be on to something…
BTW, those rodents can also do hind gut fermentation:
Hindgut fermentation is a digestive process seen in monogastric herbivores, animals with a simple, single-chambered stomach. Cellulose is digested with the aid of symbiotic bacteria. The microbial fermentation occurs in the digestive organs that follow the small intestine, namely the large intestine and cecum. Examples of hindgut fermenters include large odd-toed ungulates such as horses and rhinos, as well as small animals such as rodents and rabbits. In contrast, foregut fermentation is the form of cellulose digestion seen in ruminants such as cattle which have a four-chambered stomach which digests cellulose.
Smaller hindgut fermenters of the order Lagomorpha (rabbits and pikas) absorb the needed levels of nutrients via their upper digestive system by reingestion of cecotropes, which are passed through the intestines and subsequently reingested for added nutrients. Coprophagy is also practiced by some rodents, such as the capybara, guinea pig and related species.
While foregut fermentation is generally considered more efficient, and monogastric animals cannot digest cellulose as efficiently as ruminants, hindgut fermentation allows animals to consume small amounts of low-quality forage all day long and thus survive in conditions where ruminants might not be able to obtain nutrition adequate for their needs. Hindgut fermentors are able to extract more nutrition out of small quantities of feed. The large hind-gut fermenters are bulk feeders: they ingest large quantities of low-nutrient food, which they process for a longer time in a much longer intestine. The main food in that category is grass, and grassland grazers move over long distances to take advantage of the growth phases of grass in different regions.
Hindgut fermenters are subdivided into two groups based on the relative size of various digestive organs in relationship to the rest of the system: colonic fermenters tend to be larger species such as horses, and cecal fermenters are smaller animals such as rabbits and rodents. However, in spite of the terminology, colonic fermentors such as horses make extensive use of the cecum to break down cellulose.
So humans are more closely related to the cecal fermenters than to the colonic fermentors. Which is a good thing as I’d rather not have hooves to deal with or a large gassy belly…
At any rate, now you know…
But just in case someone thinks I’m making up this hindgut fermenter connection:
A Tale of Two Caecae…Howler Monkeys & Spider Monkeys
2 sympatric species (Barro Colorado Island, Panama)… both 7-9 kg… most of year they both eat a mix of leaves and fruit, but during food shortages, Howlers depend heavily on leaves (85% of feeding time/day, vs 50% other times of year), while spider monkeys are very frugivorous (typically 83% of time eating fruit, but during transition season, switch to only 60% fruit & more diverse diet)
Gut passage rates: average first appearance of markers…
Howler monkeys = 20 hours… max of 72 hours!
Spider monkeys = 4 1/5 hours… most passed in 8 hours
Milton’s study of ape/human gut passage rates also show humans very fast
Chivers and Hladik plot the proportions of potential fermentation surface area (stomach + caecum + colon) vs absorbtion (small intestine surface area) and find this is a good predictor of whether animals adapted as folivores, frugivores or faunivores…
While some enzymes in saliva begin breaking down carbos, most simple carbohydrate digestion / energy absorbtion takes place in small intestine
Most mechanical breakdown of food into small particles occurs with tools, teeth or in the stomach
Stomach acids can break down some muscle tissues and enzymes break down peptide bonds in proteins (more about this next time). Sacculated stomachs isolate the bacterial fermentation from the normal stomach function
Bile acids and salts (e.g. detergents) help break down lipids in the colon and fatty acids are absorbed through cells lining the colon. The caecum is a chamber attached to the colon where hind-gut fermentation can take place, and is very large in many hind-gut fermenters. Note that the human appendix is essentially a vestige of the primate caecum.
So we’re more optimal for eating fruit than leaves… but can do OK on leaves if we need to; and have adapted lately ( in evolutionary terms) to very rapid absorption of very nutrient rich foods. We are getting a much larger sized small intestine and much smaller amount of large colon.
Due to a colonoscopy when I was not effectively ‘put under’ I can report that I have a much longer colon than most (which is still way less than most other primates) which implies that I’m a more ‘primitive’ sort who can live on less nutritious and more coarse foods. (Though not by a whole lot, I’m still way short of the rest of the primates). This also matches what I know about residency times and some other aspects of digestion. That, and I can maintain my weight with very little total food intake. Trying to lose weight by dieting just doesn’t work for me. (Oddly, an “all you want to eat” while doing little other than sleep resulted in the lowest body weight during my entire adult life – 156 pounds… Normally I’m about 220 lbs or 100 kilos. It seems that if I’m in a ‘nearly hibernating’ environment, I don’t get hungry much and just use what I’ve stored…)
Such are the things you can learn by looking at things like percentage of surface area of digestive organs that are in the small intestine vs large and by looking at close relatives and how they digest food. Yes, I’ve wondered just a little bit if those of us with a touch more Neanderthal Genetics might be a bit closer to our primate paste, able to eat wider ranges of rougher foods, more efficient in digestion, and perhaps with a latent hibernation metabolism that shows during times of starvation… but all that is speculative. As, BTW, have others. There is a thesis that the Neanderthal hibernated to get through the depths of frozen winters. It’s hard to explain how they could get enough calories otherwise. But if they tended to put on a lot of extra weight when times were good, say from a fructose trigger saying to the metabolism “It is spring, start storing fat” as fruit was available; and if they could efficiently digest a wide range of foods, from leaves and fruits to meats and nuts, storing the excess; but easily burning it up when sleeping most of each winter day; well, that would explain a lot…
Things like my being at my heaviest weight when doing full workouts every other day. ( Karate class where the ‘warm up’ included over 100 sit-ups of different sorts… plus all the rest THEN class would start). Tending to gain weight most when eating a balanced diet, but losing it when doing a lot of bed rest and eating a rich high meat diet with dried grains… And why I’m just not a very picky eater. If it’s green, a fruit, nut, or has fins, feathers, or fur; odds are I’ll eat it. But I’ve also eaten frogs, so ‘feathers and fur’ are optional ;-)