What is the difference between early vedic period and later vedic period?

The early Vedic period is marked by the infiltration of
Aryan peoples into the Indian sub-continent and their interaction with the Dravidian
people. Aryans spread into the Ganges River valley about 1000 B.C.E. About that time,
they developed the use of iron tools and weapons. They used iron axes to clear forests
for agriculture; and as their agricultural practices flourished, their population grew
immensely. As their populations grew, their political structure evolved also. The local
chiefdoms became kingdoms ruled by kings in permanent cities. These kings depended on
the services of professional administrators to handle the day to day tasks of
governance. Still, they did not establish large states. Only in the 4th century B.C.E.
did any Aryan state equal the size of Harappan
society.

During the early Vedic age, the Aryans placed
substantial reliance on sacrifice of animals to their gods. It was believed that during
sacrifice, the gods visited earth, and joined worshipers in eating and drinking. Since
the presence of the gods was deemed beneficial, sacrifice became almost non-stop. A
proper household would have the Brahmins offer sacrifices not less than five times per
day; a process that was expensive and time consuming.

In
time, the practice grew old and the people disenchanted. A number of people began
retreating to the woodlands to live as hermits and contemplate the relationship between
people, the world, and the gods. A number of them were inspired by Dravidian practice.
The Dravidians had worshiped spirits associated with fertility and the generation of new
life. They had also believed that human souls took on a new physical form after death,
either as another human, or even as a plant or animal.

The
combination of Dravidian and Aryan religious ideas culminated in the
Upanishads, (literally, "sitting in front of," as a student sits in
front of a master to receive instruction,) a group of religious works that appeared over
a period of time. The Upanishads taught that individual humans were in fact part of a
greater universal soul known as the Brahman. The Brahman was
unchanging and universal, whereas human existence was in a constant state of flux. The
individual soul lived in a cycle of reincarnation, in which he would die and be reborn
as another person, animal, or plant. This reincarnation was known as karma.
This cycle was not completely desirable, as it involved a continuation of the
suffering and death all humans encountered. The ultimate goal was to break the cycle and
enter into a permanent union with Brahman, sort of a "heavenly
state."

The teachings of the Upanishads either purposely or
inadvertently justified the caste system, as one in a higher caste was believed to have
lived a virtuous life in his previous existence, and vice versa. They spoke against
gluttony, vice, materialism and failure to consider one’s relationship with Brahman; and
also encouraged personal integrity. A healthy respect for all living things, animal and
human, was also encouraged. Even though animals represented souls who had suffered from
their past life, they should not be caused additional suffering; therefore a vegetarian
diet became the norm for all who practiced the religion.