There are mainly two types of baby birth in animals.
Some lay their eggs with little or no other embryonic development within the mother. This is the reproductive method of most amphibians, fish, birds, reptiles, and monotremes. In others, like humans, there is a development of the embryo inside the parent’s body, eventually leading to live birth.
So, why is there a distinction? Why do some animals give live birth, and others lay eggs?
The egg indeed came first – the chicken and live births came later.
Over a couple of million years ago, every moving creature on planet earth laid eggs and gradually, they evolved to giving birth to live babies based on what would help them survive best. When animals evolved, the delivery method of baby evolved depending upon the external conditions.
Most mammals develop their baby inside the body. Mammals opted out from laying eggs during the course of evolution. Why? Because their developing offsprings are susceptible to bad weather and hungry predators. It is dangerous to keep your baby dug inside an egg where hungry snakes are crawling around, you see?
If birds want to fly, they have to keep their stomach empty. Babies are rather heavy to carry around, and so the birds still lay eggs rather than carrying their offspring inside. Mother birds cannot fly with weighty growing embryo passengers. Birds make it up by providing wholesome parental care, and nest construction for their offsprings.
Reptiles can be divided into three major categories: lepidosaurs (lizards and snakes), crocodiles, and turtles. Of the three, live birth is only seen in lizards and snakes. Even among lepidosaurs, most lay eggs that hatch into young, but some give birth to live young. Reptiles that give birth to live young are called viviparous. Viviparous vertebrates retain their fertilized eggs in their body until the offspring is ready to be born. In viviparous reptiles, eggs mature in the animal’s oviduct until they appear as baby versions of the adult. Egg-laying reptiles lay their eggs once the embryos are nearly one-third of the way through their growth. Viviparity is more common in snakes than in other reptiles — occurring 20 percent of snake species. This includes pit vipers, boas, and spitting cobras. Evolution blessed reptiles into different choices. There is a correlation between high altitude, colder climates, and the kind of baby reptile is producing. If a reptile is living in cold conditions, it is nearly impossible to keep an egg warm, and so they’re accustomed to keeping the baby inside their body. If a reptile is living in warmer conditions, the body cannot stand weight, so they lay eggs.
Millions of years ago, everybody laid eggs. As time passed and animals evolved, some chose to keep laying eggs as babies were tough to carry, and others decided to develop an embryo inside the body to provide safety and warmth to babies.