Germanic criminal codes had something called "compurgation," where a person accused of a crime could be found innocent if a group of "oathhelpers" (usually 12 or 25 men) were willing to swear an oath that the accused was innocent. If later it turned out to be that the accused was guilty, the oathhelpers were in danger of being punished for the crime. From Sir William Searle Holdsworth, A History of English Law: The Judicial System (1922), 1:305-6:
If a defendant on oath and in a set form of words* will deny the charge against him, and if he can get a certain number of other persons (compurgators) to back his denial by their oaths, he will win his case. If he cannot get the required number, or they do not swear in proper form, " the oath bursts," and he will lose. Though oaths were used in the Roman law of procedure, this institution of compurgation was not known to it. It was, however, common to the laws of many of the barbarian tribes who overran the Roman empire. Because it was so common and so widespread the church adopted it. Churchmen who could command the services of their fellow ecclesiastics as compurgators found it to be a system "admirably suited for their defence in an age of brute force."
According to the older formulas the compurgators took the same oath as their principal. They swore that he did not owe the debt, or that he was not guilty. And therefore they were liable to the penalties for perjury in just the same way as their principal. But, in the course of the twelfth century, it came to be thought that the compurgators should only be required to swear to their belief in the truth of their principal's assertions ; and to this opinion legal sanction was given by Innocent III. This was due to a wish to remove the temptation to commit perjury; but it destroyed much of the efficacy of this method of proof, because it prevented any effective punishment of a compurgator who swore falsely. Partly for this reason, partly because the revived study of the civil law was teaching men more modern ideas about procedure and evidence, compurgation about this period began to be looked on with disfavour. But it still had many centuries of life before it, and it was in the two succeeding centuries that precise rules were laid down by the common law as to the forms which must be used in carrying out this process. For instance, there does not seem to have been originally any certain rule as to the number of compurgators required. Three to six compurgators were generally thought sufficient in the manorial courts; and Fleta thought that their number should always be double that of the secta. It was not till 1342 that it was settled that the number must be twelve.In the early days of the jury system, jurors were your peers who knew you, and would judge whether you could have done the crime based on their knowledge of your character. Today, of course, it is quite the opposite: no juror would be selected if he knew the defendant, because then his knowledge of the defendant's character would impair his ability to make a judgment of the facts.