This week on SciShow News, scientist have taken some steps to solve a couple genetic mysteries: why birds lay eggs, and biology of addiction.

Giving birth to live young, called viviparity is a pretty sweet evolutionary strategy. It seems to be helpful in colder environments, and predators might gobble up eggs more easily than little scampering offspring. Lots of vertebrates give birth to live animals, including most mammals and a number of snakes and lizards. But there's one huge exception: birds, and larger group they belong to called archosauromorphs, which includes crocodiles and some dinosaurs. 

Until this week, we thought all these creatures had always laid eggs. Maybe because of some genetic reason that prevented viviparity. But according to a new study published in the journal Nature Communications, we were wrong.

The study is about an archosauromorph called Dinocephalosaurus, a marine reptile dating back to the Triassic period. It's a primitive member of the group, only distantly related to crocodiles or birds. But these researchers think that it gave birth to live young. As evidence, they presented a fossil from around 245 million years ago with and embryo clearly preserved inside of it. The tiny bones look like a mini-adult. 

There's no sign of fossilized calcium material from an eggshell, so it probably wasn't in an egg. And the researchers know it's not something the adult specimen ate, because it's in the wrong position to be food. It's facing with its head forward, while most marine predators swallow their prey head-first, so it would be preserved tail-forwards. Plus, the baby is curled up like a typical vertebrate embryo. It's in the fetal position, or as close as you can get for a reptile whose neck is longer than its body. 

So whatever's stopping birds and crocodiles from giving birth to live young didn't stop their earliest relatives. Maybe flight makes it impossible for birds, since they'd have to carry around those heavy, growing offspring. Or maybe there was some other evolutionary pressure for archosauromorphs to lay eggs. Scientists don't know for sure yet, but this study means there are making progress. 

Meanwhile, scientist do know that addiction is at least partly genetic, and some people are more predisposed to form drug habits. And in a paper published this week, researchers from the University of British Columbia have genetically engineered mice that can resist cocaine addiction. 

Their study involved a group of proteins called cadherins, which help your cells stick together. Cadherin also helps with the formation and stability of synapses, the gaps between your nerve cells, which your brain uses to do everything from sending messages through your body to storing memories.

Addiction generally involves intense memories. Cocaine molecules, for example, cause a buildup of extra dopamine, a neurotransmitter associated with feel-good reward systems in the brain. So cocaine addiction changes synapses in that reward system, to build memories of those euphoric highs, which can make people change their behavior and keep seeking out that feeling.

In this study, researchers genetically engineered mice to produce extra cadherin, which they predicted would mean more stable synapses, stronger memories, and more tendency towards addiction. But what they found was the opposite: mice that were producing more cadherin showed less tendency toward cocaine addiction.

Their experiment involved a well-established method called conditioned place preference, where a box is divided into rooms that are different enough that mice can tell them apart. The walls of each room can be decorated differently, the floors can have different textures, there might be different smells, and -in this case- one of them was associated with a dose of cocaine. 

In this experiment, the mice were placed in a room with plain walls while they were high on cocaine, and the other room had stripey walls. After a couple of days of this, the mice were put in the box without any drug and allowed to wander. The normal mice spent more time in the plain-walled room, possibly because their reward systems had been tricked into remembering that this was an awesome place to be. Not that cocaine is awesome. But the mice with the extra cadherin spent the time in both rooms. And the researchers took this to mean the genetically-engineered mice weren't forming strong memories related to addiction. It's possible that, instead of helping create or strengthen synapses with related memories to cocaine, the extra cadherin prevented existing synapses from being changed.

Now, the researchers caution that this finding isn't very ready to apply to humans yet. Our synapses need to get stronger and weaker, or reform completely for our memories to work properly, so flooding a human brain with extra cadherins isn't something we would want to do.

What it does help with is understanding which genes might play a part in addiction. That can help scientists identify people who might be more predisposed to addiction, or others who might be more resilient, which could eventually help us develop biochemical treatments.

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