SAN FRANCISCO -- Quarterback Alex Smith won't play in the 49ers' showdown against the Chicago Bears tonight because of a Nov. 11 concussion, the 49ers have confirmed.

Smith practiced the past five days but did not gain further medical clearance from the team's medical director, Dr. Dan Garza.

Second-year understudy Colin Kaepernick will make his first career start, and he'll face a defense that leads the league in takeaways and ranks second in fewest-point allowed.

The Bears also will be without their starting quarterback, Jay Cutler, who also sustained a concussion Nov. 11. Former Raiders quarterback Jason Campbell will start in place of Cutler, who was ruled out Friday and did not practice all week.

Both the 49ers (6-2-1) and Bears (7-2) are jockeying for playoff position and the winner will have an inside track on a potential first-round bye.

Smith reported blurry vision, nausea and headaches from a concussion that knocked him out of a 24-24 tie against the St. Louis Rams. Smith has started the past 28 games, including playoffs.

Earlier in the week, Smith endured his normal workload in practice and showed no signs of lingering effects from his second documented concussion, according to coaches and players. He also sustained one in last season's home opener but practiced and played the following week's game, at Cincinnati.

Coach Jim Harbaugh said Friday that Smith was progressing well with the NFL's post-concussion protocol but that the decision to play him would be in doctors' hands.

John York, the 49ers' owner, chairs the NFL Owners Committee on Health and Safety, and he took part last month in the fourth International Consensus Conference on Concussion in Sport in Switzerland.

Kaepernick saw sporadic playing time in five games this season before being summoned to replace Smith in the second quarter of the Nov. 11 tie with the Rams. He completed 11 of 17 passes for 117 yards and led the 49ers to a fourth-quarter comeback to force overtime.

For the season, Kaepernick is 16 of 26 for 206 yards with no touchdowns and no interceptions. Kaepernick's legs are perhaps a bigger threat than his arm, especially for a 49ers offense featuring the league's No. 1 rushing attack. Kaepernick has 21 carries for 177 yards (8.4 average) with three touchdowns.