Yes, you share a common ancestor with pigs, but it was a long, long time ago.

I was recently invited to comment on an Anthropology Network discussion on LinkedIn, where someone asked, "I'm wondering what this community's thoughts are about the theory that humans are a hybrid?" and linked to the blog post by Eugene McCarthy supposing that humans resulted from a hybridization event between chimpanzees and pigs. Because it is a private network, I'd like to repost, with some expansions, what I added to that discussion. But, let's just start off by clearing the air: 

Chimpanzees did not mate with pigs and produce humans.

Chimpanzee, by Ikiwaner (Own work), via Wikimedia Commons

Adorable piggy, by A R, via Wikimedia Commons

I am not a little piggy!

I was surprised that a group of anthropologists would even consider this a reasonable topic for discussion, but perhaps this speaks to the need for more cross-discipline communication. 

I find it distressing that the chimp-pig hybrid article dismisses the abundance of genetic evidence that provides no support for such a hypothesis of human ancestry. There are several scientific and logical flaws with the supposition that humans resulted from what must have been multiple matings between chimpanzees and humans. Also, please see an alternative discussion of this hypothesis by PZ Myers here, then another summary and discussion by ARTIOFAB here. 

1. Humans are genetically very similar to chimpanzees, and genetically distant from pigs
There are no regions of our genome where the genomic content more closely resembles a pig than a chimpanzee. If such a hybridization had occurred, we would, like we do with the Neandertal and Denisovan genomes, find regions where segments of modern humans are more closely related to pigs than any other species, but we do not. 

I really don't understand how anyone can look through images of pigs and think that we resemble pigs more than we do chimpanzees or bonobos:

Pan paniscus (bonobo) By Pierre Fidenci (http://calphotos.berkeley.edu), via Wikimedia Commons

2. Body hair has been lost independently in many mammalian lineages.
Yes, hairlessness over most of our bodies evolved in humans, but it did so independently in the human lineage. Similarly hairlessness independently (we call it convergent evolution) evolved in naked mole rats, manatees, and cetaceans (dolphins and whales). Rodents are actually more closely related, evolutionarily, to humans than pigs, but we don't see a naked mole rat - chimp hybrid theory because it is obviously ridiculous to the general public. The chimp-pig hypothesis is even more improbable. And, furthermore, many pigs have not lost hair on their bodies or faces.

Bearded Pig, by Art G. from Willow Grove, PA, via Wikimedia Commons

3. Chromosomal and genetic differences between chimpanzees and pigs preclude fertile hybrids.
The chromosomal differences between chimpanzees (48) and pigs (38) would preclude any chimp-pig zygote from developing, or even replicating properly. The author greatly over-exaggerates claims about the fertility of hybrids of sheep-goat hybrids (most are stillborn), and also misuses the term geep (which refers to a chimera of sheep/goat cells). The author also ignores the close evolutionary relationship of sheep and goats, where the chromosomes (and the breaks/fusions) can readily be mapped, where most of the gene content is still conserved among chromosomal regions. Such an identity of the order and orientation of gene content does not exist between chimpanzees and pigs, but is possible between the very closely related human and chimpanzee (all chromosomes are one-to-one, except for human chromosome 2, which is a fusion of two ancestral chromosomes that remained unfused in chimpanzee). 

Mapping of human and chimpanzee chromosomes, by JWSchmidt, from Wikimedia Commons

4. Fossil evidence can account for all of humans ancestry to the human-chimp common ancestor.
At what point in time would this have occurred? Certainly not the present. We have fossil evidence of modern humans, ancient humans, ancient hominids, ancient apes, ancient monkeys, and so on. Science can account for the progression of humans from our shared ancestor with chimpanzees, and even further back. At no point in history do the fossils of ancient humans in any way resemble the fossils of ancient pigs. 

5. Modern species share common ancestors, they did not beget each other.  
Modern humans did not evolve from modern chimpanzees any more than modern chimpanzees evolved from modern humans (that is, not at all). Fossil evidence suggests that the chimp-human common ancestor looked a lot more like a modern chimpanzee than it did a modern human, suggesting many more physical changes along the human lineage, but the modern chimpanzee has also experienced changes since our most recent common ancestor together, approximately 6 million years ago. Both humans and chimpanzees share a common ancestor with pigs about 90 million years ago.

6. Domesticated pigs and chimpanzees do not live in the same locations. 
Pigs (with reduced body hair) were domesticated in East Asia and in Europe. Chimpanzees live in central Africa. They live on different continents. They did not ever have the opportunity to get busy. Wild boars can be found in northern Africa, but this is still quite far from where chimpanzees live, and if they do overlap in range, it is only a very recent occurrence.

Wild Boar, by Volker.G (Own work), via Wikimedia Commons

7. Why hasn't an experiment been done? 
You could attempt each of the possible combinations (chimp sperm with pig eggs, and pig sperm with chimp eggs) and test the viability. It it works, we can have this discussion. If not, this guy needs to stop spouting nonsense that detracts from the real science being done.

By David.Monniaux, via Wikimedia Commons

Why sequence the manatee genome?

By Gaylen Rathburn, via Wikimedia Commons

I was excited to learn today that there is genome sequence for the West Indian manatee (Trichechus manatus)!! A friend wondered why I was so excited, asking, "Is it evolutionarily interesting?"

Well, first off, I've always been fascinated with manatees: they are so defenseless, and yet grow so large that they are rarely predated on. They are, however, especially susceptible to human-made water vessels. I am excited for their genome because I've always loved them. But, they are also pretty evolutionarily awesome.

1. Manatees are more closely related to elephants than they are to dolphins or whales.
The manatee and the bottlenosed dolphin are approximately 100 million years diverged from each other. The manatee is classified under the Afrotheria, which also includes elephants, hyraxes, and aardvarks, while the dolphin is classified as Laurasiatheria, which also includes the red panda, hippos, horses, rhinoceros, and bears. You can click on the links above for more detailed lists of the species included in each group.

Although about 100 million years separate manatees from dolphins, the manatee and elephant are only separated by about 61 million years. Unlike dolphins, but like elephants, manatees have toenails:

By Fritz Geller-Grimm, via Wikimedia Commons

2. Manatees convergently (independently) evolved the ability to live under water
Unlike other mammals that live primarily in the water (whales and dolphins), the manatee (and dugong) does not breath air through a blowhole on top of its head. Instead, manatees breath through their nostrils. How cool is that?

By Rusty Clark from Merrit Island FL via Wikimedia Commons

Additionally, manatees independently evolved flippers and wide tails.

Manatees also convergently lost most of their hair (although this isn't unique to water-dwelling mammals as many terrestrial mammals lost most of their hair too, including pigs, naked mole rats, and humans). I wonder whether similar genes are involved (disrupted?) to result in the loss of hair across these mammals?

3. Manatees have a much lower metabolic rate than expected for their body mass.
Manatees are fairly sedentary, and have a low metabolic rate (0.36 times the predicted rate for placental mammals). Dolphins have a much higher metabolic rate, but a similar expected lifespan as manatees (approximately 50-60 years, to my understanding). So, it will be fascinating to investigate how genomes differ between fspecies with similar generation times, but very different metabolic rates.

Platypus, platypus, platypus, platypus!

Platypuses at Berkeley's Museum of Vertebrate Zoology

The platypus is currently tied for my favorite mammal (along with hedgehogs and manatees). Platypuses have a lot of unique characteristics, but one of the features I find most fascinating is their sex chromosomes. Before a post about their chromosomes, there's a few things we need to clear up.

1. The platypus is not a "cross" between a duck and a beaver.
Because of its unique features, there is a lot of confusion about the platypus. The platypus is not some strange hybrid. A duck and a beaver cannot produce an offspring together. The population of platypuses evolved, like all other living organisms.

Upon closer inspection, looking at the picture above, the platypus bill looks very little like a duck bill at all. The platypus bill is wide and flat, and appears to be more leathery than the hard duck bill.

By DickDaniels (http://carolinabirds.org/) (Own work) via Wikimedia Commons

And, even though popular cartoons continue to draw its tail as if it were beaver-like, the platypus tail is relatively short, and is covered with soft brown fur, not at all like a beaver's large hairless tail:

The beaver has a large, flat, hairless tail. By Steve, Washington, DC via Wikimedia Commons

2. The platypus is not the ancestor of modern mammals, it is a modern mammal. 
Although it lays eggs, and doesn't have breasts or nipples, the platypus is still classified as a mammal. Platypuses are part of the group of egg-laying mammals called "monotremes." These are not "proto-mammals." Nor are they "primitive". Monotreme mammals have been evolving for the same amount of time as all other mammals. As humans we share a common ancestor with platypuses, approximately 220 million years ago. That doesn't mean that it isn't useful to understand more about the platypus, but interpretations should be careful not to assume the platypus has maintained the ancestral state of all mammalian traits.

3. The platypus is not the only egg-laying mammal. 
In addition to the platypus, there is another group of monotreme mammals that lay eggs: Echidnas. Echidnas and platypuses diverged from one another about 64 million years ago. While they share some characteristics that are unique to monotreme mammals (relative to other mammals), such as egg-laying and oozing milk out of mammary pores instead of having nipples, the two groups of species have accumulated many differences. Perhaps one of the most notable is that there are at least four species of echidna, and only one species of platypus.

Other cool echidna features include their body covering which includes a mixture of course hair and dense, pointy, spines.

Echidnas at Berkeley's Museum of Vertebrate Zoology

Echidnas also have long durable nails that they use for digging in the sand and dirt.

Check out those nails.

4. Platypuses are about the size of a house cat
I don't know why, but when I was growing up, I always imagined that platypuses would be fairly large critters - not unlike a recently discovered branch in the platypus tree that went extinct 5-15 million years ago. Turns out, modern platypuses are actually about the size of a house cat. You can see the pictures below  with my hand next to them.

Not so giant platypus.

If you want to see a live platypus (which I really, Really, REALLY do!!), check out this video of some people hand feeding a platypus.

What makes a mammal?

Cross-posted at PandasThumb.

I saw a tweet wondering about what makes an animal a mammal:

@realscientists If a tiger shark's eggs hatch inside it and are born live (I saw a pic recently) is it a mammal or a fish? :-)
— Fiona Howie (@MummyFiFi) September 29, 2013

So, I thought I'd go through a few of the common ideas about shared physical features of mammals.

What makes a mammal? 
Is it giving live birth? Or having hair/fur? What about feeding their babies milk?

Well, kind of (I'll tell you at the end what really does). First, let's go through these three:

Live birth.
Not all mammals give live birth. Monotreme mammals including the echidna:

Short-beaked echidna. Photo by Fir0002/Flagstaffotos

and platypus:

Swiming Platypus. Photo by Peter Scheunis

don't give birth to live young; They lay eggs.

Okay, so not all mammals give birth to live young.

Hair.
What about having hair/fur. All mammals have hair/fur, right?

Well, I suppose technically baby dolphins have whiskers, but you wouldn't know it from the adults.

By U.S. Navy photo by Photographer's Mate 1st Class Brien Aho. [Public domain], via Wikimedia Commons

Pangolins, with their armor-like plates, actually have a little bit of fur on their underside (although it'd be hard to tell):

By Valerius Tygart (Own work) [CC-BY-SA-3.0 or GFDL]

And naked mole rats technically aren't completely naked, they also have whiskers (but they are mostly naked):

By Ltshears - Trisha M Shears [Public domain], via Wikimedia Commons

So, we'll give hair a maybe. Mammals do have hair, but there are several cases where one might mistake a mammal for not having hair.

Milk.
Do all mammals feed their babies milk?

By Trish Steel [CC-BY-SA-2.0], via Wikimedia Commons

First, the snarky answer - No. Male mammals generally do not make milk (although they could).

By User:Ltshears, edited by User:julielangford [Public domain]

Second, more serious answer - Yes. I don't know of any species of mammal that nourishes their offspring with anything other than milk. To learn more about mammals, lactation, and milk, check out the blog "Mammals Suck (... Milk)" by Dr. Hinde.

Breastfeeding. By honey-bee [CC-BY-2.0]

Actually, what makes a mammal is more than just whether they give live birth (because not all do), and have hair, and lactate. And doing each of these things does not necessarily mean the animal should be classified as a mammal. (Note: Although it gives live birth, a tiger shark is not a mammal, it is a shark; sharks are a kind of fish.)

So, what does make a mammal?

Shared evolutionary history
Mammals are a group of species related by their evolutionary history. The picture below is a phylogenetic tree showing the evolutionary relationship between many different species.

CT Amemiya et al. Nature 496, 311-316 (2013) doi:10.1038/nature12027

All mammals share a common ancestral population.

Modified from: CT Amemiya et al. Nature 496, 311-316 (2013) doi:10.1038/nature12027

The classification of "Mammals" was made based of shared physical and anatomical characteristics. But, underlying those, is a shared evolutionary history.

We do make sub-divisions within that larger grouping of mammals. For example, the egg-laying mammals, platypus and echidna are called "Proto-theria", while all other mammals are called "Therians". There are many other sub-classifications, but they are all still part of the broader group of mammals.

There are also larger groupings. For example, on the picture above you can see all the species highlighted in pink are called Tetrapods. These are all descended from a common ancestral population of tetrapods that are generally four-limbed vertebrates.


Although their physical characteristics may change, all species that descended from the common ancestral mammal population will all be mammals.

So, how do you tell what a mammal is? 
Well, the broad rules of thumbs still apply. If you are a Naturalist, roaming through some uncharted region, and you happen across an animal you've never seen, you can start with some of the general physical characteristics (e.g., is it warm-blooded? does it have fur?). But, now you can also take a look at it's DNA as another line of evidence.

You can collect and sequence a sample of DNA from hair, or blood, or a toenail, or even from scat (aka poop), then compare the sequences you find with sequences that are already available to learn more about the creature you sampled. You can build a tree (like the one above) based on the similarity between the sequences. The relationship between the sequences for any one region or gene may not reflect the broader species tree, but it will give you an idea of where your species fits. And, the more DNA sequence you analyze, the better your resolution will be come (although it sometimes happens that biology is just messy).

Mammals
"Mammals" is the term we use to describe the group of species that generally share a defined set of characteristics (warm blooded, lactating, give live birth, have fur/hair), AND share an evolutionarily recent (300ish million years ago) common ancestral population.