Most Puraan give the description of Avataar (Incarnations of God) - some in short, some in detail.
Some scientists regard them as the pointers towards evolution.

According to Shatapath Braahman
The Shatapath Braahman mentions the story of Matsya as also the Koorm and Varaah Avataar (1.8.1.1-6, 7.5.1.5., and 14.1.2.11). Although there are different versions on the Avataar of Buddha, Balaraam and Krishn, there are no doubts about about the Avataar of Parashuraam.
Vaaman Avataar is mentioned in 1.2.5.1.

Foundation of Evolution
According to Taitteerya Upanishad, which deals with primary evolution, the soul is to be shrouded with five sheaths -
The first sheath is called Anna-maya (Annamaya Kosh), means made of food
The second one is called Praan-maya (Praan Kosh), means made of Praan or breath or Vaayu
The third one is Mano-maya (Man Kosh), means made of mind
The fourth one is Vigyaan-maya (Vigyaan Kosh), means discriminatory intelligence
The fifth one is Aanand-maya (Aanand Kosh), means eternal bliss.

If interpreted,
--The first creation, of first evolutionary stage of life (anaerobic form) depended only on food for one's survival and multiplication. This is the most primitive form. Methanogenic (methane producing) or archae bacteria are in this class.
--The second one, is the appearance of aerobic forms of life that require oxygen for survival. The Upanishad calls this stage the Praan-maya sheath.
--The third stage is the appearance of intelligent forms of life. These are the insects and lower forms of animals. They have the ability to perceive and to react instinctively to their environment. They lack the ability to analyze, sort out and make sense out of the messages.
--The fourth one is the appearance of the Vigyaan-maya species or with discriminatory ability species. Collation of a lot of information and analyzing them is the special skills that this form of life has. This is the highest form of evolution. So far only human beings have this ability.
--The last evolutionary state of bliss is yet unknown. Geetaa calls Krishn as the highest, Purushottam, so He may qualify for the highest evolutionary level.

Theory of Evolution and Avataar
The 10 Avataar of Vishnu are in agreement with the idea of evolution of species held by biologists. According to them, life started as unicellular organism in aquatic environment and progressed to the higher forms. The Avataar concept is singularly Hindu concept that says that whenever Dharm decreases on Earth God descends to re-enforce it. (Geetaa, 4:7-8). According to fossil records, life has been destroyed several times and had reappeared again and again. The 10 incarnations of God can be justified on the basis on the above assertion.

(1) Matsya Avataar (Fish Incarnation) -
The first Avataar is the Matsya (Fish) Avataar in aquatic environment. God comes in the form of a fish to save life forms from a flood. The episode of the Great Flood has been found in the Mesopotamian epic "Gilgamesh" and from there it was taken by Jews and now it has spread in all religions. In order to indicate the aquatic origin of the animals, the Lord incarnates in the form of a Matsya, a fish.

The Hindu chronology is set by the date of disappearance of the river Saraswatee. The event occurred about 9,000 years ago. The riverbed can be seen from satellite photography. The river was very broad, 27 kms at its widest point. The river flowed from Maanasarovar Lake to the Arabian Sea and disappeared after a cataclysmic upheaval in the Himaalaya that blocked its course. Therefore one may be justified to say that Hindu civilization and beliefs are older than the Egyptian and Mesopotamian ones. The distinguishing feature is that Hindu civilization is characterized by cultural artifacts and not by monuments.

The absence of archaeological evidence is because the mythological age was over before the invention of burnt pottery. The method of making fire was invented by a Vaidik sage Angiraa The method is still used in the ritual known as Yagya. It is therefore very plausible that human settlements from the hunter stage started first in Aaryaa-vart or Northern India. Fire God or Agni is considered as the foremost protector (Purohit or well wisher) of humanity (from wild animals). Aaryans, Hindu and Zoroastrians, are the only people who worship Fire as a God.

(2) Koorm Avataar (Tortoise Incarnation) -
The next evolutionary stage is of amphibians - Koorm or the Tortoise, follows this aquatic origin of animals. This Avataar has more than usual significance. The purpose of this Avataar was to secure the nectar of immortality from the Heavens. Significantly the turtle has the highest longevity amongst the species. Strangely enough, Mongols also regard the Turtle as Divine. There was a huge statue of the turtle in the unfinished capital of Ghengis Khan, Karakoram. The turtle played a major part in the churning of the vast ocean of milk in the outer space. The purpose was to get to nectar. Many animal species and blessed forms of divinity emerge from the ocean. This is important because it confirms what the Padmanaabh icon implies, namely, a divine hand/participation in evolution of life. This would correspond to what creationists, mostly based in the US, believe in. They call their belief “intelligent design". A sculptured model of the churning of the ocean of milk is placed in the lobby of the Bangkok airport in Thailand.

(3) Varaah Avataar (Boar Incarnation) -
The next avatar is Varaah or wild Boar. In this stage life has emerged from the aquatic environment and is wholly independent. The third incarnation that is completely a terrestrial animal.

(4) Narasinh Incarnation (Man-Lion Incarnation) -
Varaah is followed by the lower human forms known as Homo erectus in science and in mythology as Narasinh or the man-lion. Narasinh Avataar represents a beast's attempt to obtain a human form. This species stood erect but was still governed by animal instincts. We hear of monkeys assisting Raam in the epic Raamaayan. Maybe these monkeys were the subhuman species living side by side with humans without interbreeding. There is evidence that in an Israeli cave the Neanderthals and the humans lived without interbreeding for 50,000 years.

(5) Vaaman Avataar (dwarf Incarnation) -
The midget human Vaaman is the next stage in mythological evolution. In Vaaman Avataar, a pigmy human being is represented.

(6) Parashuraam Avataar -
In Parashuraam Avataar, a human being's tremendous physical strength could be recognized.

(7) Raam Avataar (Raam Incarnation) -
From there evolution takes us to a rule-bound human being in the form of Raam. In this Avataar, one can identify perfect human qualities.

(8) Krishn Avataar -
The last stage of evolution is Krishn, the Purushottam or the perfect human being. In this Avataar, Vishnu is fully mature to handle various kinds of situations with His intelligence. This is why Krishn is worshipped in all stages of life - as infant Mukund, as a child Daamodar or Baal Krishn, as a carefree teenager, Venu Gopaal, as a lover Raadhaa-Krishn, as a warrior (Kans Chaanoor Mardan), and finally as the philosopher on the battle field of Kuru Kshetra. He represents the quintessential joy of life. The Geetaa devotes one full chapter, Chapter X, to evolution in all species. It concludes that all forms of excellence are divine manifestations.

(9) Buddh Avataar -

(10) Kalki Avataar (Kalki Incarnation - yet to come) -
Kalki Avataar represents our own destruction of giving scant attention to our fauna and environment.