(Brief) Christian view, according to the Bible (if you're curious ;D)... Many answers to your question in a letter written by Paul to a church in Rome around 50-55 A.D. Check out just the first chapter:
Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God, (Which he had promised afore by his prophets in the holy scriptures) concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh, and declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead.
--Paul was a Jew who converted to Christianity (Acts 9). He followed the Old Covenant, or contract, between Yahweh (YHWH) and the Jews, specifically Abraham (Genesis 13). As indicated above, Jewish prophets spoke on behalf of God, and what they said was recorded as part of the Hebrew Bible, or "holy scriptures" (first 39 books of Bible). According to these scriptures, God not only existed, but was present with them from Creation (Genesis 1-2, written by Moses according to the Bible). Because of the Fall of Man (Genesis 3), sin and death entered the world. In order to abolish these things, God promised to send someone to pay the penalty for man's sin. This is the 'gospel,' or good news, that he speaks of in Romans, that Jesus Christ of Nazareth (Matthew, Mark, Luke, John) was made "of the seed of David," as prophesied in the scriptures (meaning his lineage can be traced back to Israel's King David), and "declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead." As the New Testament indicates, Jesus Christ was crucified, then appeared alive to the disciples and others after his death, conquering sin and death (sin=death, so if you do not sin, you conquer both according to the scriptures... Jesus is the only one to have ever done these things on his own). His resurrection is the "sign of Jonah," who was swallowed by a big fish only to escape death three days later, that Jewish leaders were told by Jesus would be God's sign to them of the Messiah.
That is the biblical proof, which many believe can be confirmed through other means such as archaelogy. As to why scientists think mankind came from apes, continue reading the chapter (verse #'s included):
16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
17 For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness;
19 Because that which may be known of God is manifest in them; for God hath shewed it unto them.
20 For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:
21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
22 Professing themselves to be wise, they became fools,
23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
24 Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves:
25 Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
26 For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature:
27 And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet.
28 And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient;
29 Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers,
30 Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents,
31 Without understanding, covenantbreakers, without natural affection, implacable, unmerciful:
32 Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them.
--- According to Paul's view, as a result of men not putting their "faith" (moral conviction of the truth) in the gospel of Christ, they consequently have their "foolish heart... darkened," or given over to their "lusts" (desires). Because men became "vain in their imaginations," they thought up things, one being that the "glory of God" is equivalent to a "four-footed beast," such as an ape. "The glory of God," is reflected by men according to the scriptures. Simply put, Paul is arguing that if people do not believe the gospel of Jesus Christ, they will fall further into sin while believing and making up lies.