Unexplained Biology? How Humans Are Similar To Sealions

Human babies and sealion pups both share the same unique adaptation, why?

If you’ve ever been present at the birth of a baby you may have noticed something. Babies often enter the world coated in a kind of waxy, watery stuff. It feels a bit like lard, and after it is washed off you never think about ever again. This fatty layer on the newborn skin is called the vernix caseosa - a rather unpleasant term which translates as cheesy varnish. The intriguing thing is that no-one knows what the vernix is for. Even stranger than that, humans are the only terrestrial mammal that produces vernix on its young. This raises a whole host of questions, but I will add the final oddity into this mix - the only other mammal which does produce vernix (that we know of) are sealions.

A newborn with vernix visible on its skin

To break this down we need to start by describing the vernix itself. Its a sort of membrane made of roughly 80% water, 10% proteins and 10% lipids, or fats. Embedded within the layer are a type of skin cell called corneocytes. How the vernix is produced is still something of a mystery, but it appears that around 21 weeks, the baby’s skin begins developing into its final exterior layers, a process which leaves sebum and free cells on the surface - these eventually emulsify into the vernix layer. Strangely there also seems to be some kind of interaction between vernix and pulmonary surfactant compounds which both emulsifies the vernix and may assist in changing the composition of the amniotic fluid. What is clear is that vernix is understudied, and much more research is needed to work out what is going on.

Focusing on the lipids presents yet another set of mysteries. Lipids are a highly diverse class of molecule, and can be modified for very precise purposes. The most basic saturated fats are simply long links of carbon and hydrogen with an acid group at the end. Unsaturated fats possess one or more double-bonds, which gives them more physical flexibility, which is why they are often liquid at room temperature. Vernix possesses some unusual lipids, such as cholesteryl esters of w-(O-acyl)-hydroxy fatty acids, but most bizarrely the lipid composition of the vernix is identifiably different between males and females, a result which may arise from different hormone patterns in the baby.

The precise function of the vernix is again, not well understood. Different studies have found support for:

• Waterproofing

• Anti-bacterial and anti-viral defence

• Lubrication for delivery

• Moisturising the skin

• Thermoregulation of the baby after birth

• Speeding up wound healing

None of these have been definitively proven, and the fact remains that the appearance of the vernix is not even or consistent. Some babies are born with lots of vernix and some born with almost none. There are no studies to the best of my knowledge looking at vernix distribution by weight, health, maternal status, ethnicity, diet or other external factors.

Until recently it was believed that only humans produced vernix, but this all changed when researchers began identifying it in deceased California sealion pups. An algal bloom in 2013 in southern California resulted in many sealion stillbirths and abortions, offering a rare opportunity to examine the deceased newborns.

Chemically the vernix of the sealion is similar to that of humans, rich in long branched-chain fatty acids and a compound called squalene. This is where it continues to get strange, so just bear with me.

Squalene is a triterpenoid compound first identified in sharks, in their livers. It has an important role in hormone production, but it is also present on the surface of human skin, in much greater concentrations than areas which use it for manufacturing hormones. In a survey of 60 mammals, only four were found to have squalene on their skin: the nocturnal arboreal rainforest dwelling kinkajou, the mole, the beaver and the otter. All four of these live in moist, damp environments. Humans and sea lions bring the total to six mammals.

So to recap - humans at birth are covered in a waterproof biofilm, something found in only humans and California sealions. Human skin after birth is also high in squalene, something only found in five other aquatic or moisture-adapted mammals.

What is going on here?

Proponents of the ‘Aquatic Ape’ hypothesis - the theory that humans evolved with a suite of traits adapted to shoreline foraging and extended periods of time in and around water - point to the vernix and to squalene as evidence for the idea. Certainly the overlaps are curious, humans sharing biomolecular features only with other marine or damp-loving animals.

It could be that human hairlessness and the need for additional heat after birth has prompted the formation of vernix, but there are many other hairless mammals, and human babies already have plentiful inbuilt thermal features. The squalene/vernix coating would certainly benefit a marine or damp-living animal whose offspring would face many more pathogens in a colder, moister birth setting, but humans evolved in a warm, savannah like environment. Whatever the trigger, my guess is that other animals also possess a vernix, we just haven’t found them yet - and that the vernix belongs to a deeper and older set of mammal traits which have been switched off in most. Quite why humans re-activated or re-discovered the vernix is a mystery, and we don’t know enough about the function of it to point to any specific benefit.

The Aquatic Ape theory lurks in the background of this topic, mostly because only its proponents care about comparing humans to marine animals. It enjoys almost no academic or professional support as a hypothesis, and badly needs updating in the light of modern fossil and genetic evidence if it is to be useful. But never say never.